Method for producing γ-glutamylvalylglycine or a salt thereof

ABSTRACT

A method for producing γ-Glu-Val-Gly comprising the step of reacting Val-Gly with a γ-glutamyl group donor in the presence of a γ-glutamyltransferase, a microorganism containing the enzyme, or a processed product thereof to generate γ-Glu-Val-Gly, wherein the γ-glutamyltransferase consists of a large subunit and a small subunit, and the small subunit has a specific mutation.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of and claims the benefits of priority to International Application No. PCT/JP2012/075908, filed Oct. 5, 2012, the entire contents of which are incorporated herein by reference. International Application No. PCT/JP2012/075908 claims the benefits of priority to Japanese Patent Application No. 2011-223058, filed Oct. 7, 2011.

TECHNICAL FIELD

The present invention relates to a method for producing γ-glutamylvalylglycine or a salt thereof, and a mutant of γ-glutamyltransferase preferably used for the method. γ-Glutamylvalylglycine is useful in the fields of food, drug, and so forth.

BACKGROUND ART

A certain kind of peptides such as γ-glutamylvalylglycine (L-γ-glutamyl-L-valyl-glycine, henceforth referred to as “γ-Glu-Val-Gly”) have a calcium receptor agonistic activity (Patent document 1). Such peptides having a calcium-receptor activation action are known to be able to impart kokumi taste to foods and drinks (Patent document 2), improve tastes of low fat diets, especially fat-like thickness and smoothness (Patent document 3), improve feeling of body of sweet taste substances, and improve bitterness peculiar to sweet taste substances (Patent document 4).

Moreover, such peptides as described above are known to have a prophylactic or curative effect on diarrhea (Patent document 5) and diabetes (Patent document 6), and a bicarbonate secretion promoting effect in the alimentary tract (Patent document 7).

As described above, wide application of γ-Glu-Val-Gly in the field of food, drug, and so forth is expected.

As methods for producing tripeptides, chemical synthesis methods and enzymatic methods are generally known. As one of the chemical synthesis methods, a method of selectively γ-glutamylating a dipeptide by using N-protected glutamic anhydride to obtain a tripeptide is known (Patent document 10). As the enzymatic methods, there is known, for example, a method of reacting a dipeptide having an esterified or amidated carboxyl terminus and an amino acid having free amino group (for example, an amino acid of which carboxyl group is protected) in the presence of a peptide-producing enzyme to produce a tripeptide (Patent document 8).

As an enzyme that catalyzes the reaction of transferring γ-glutamyl group to a dipeptide, γ-glutamyl transferase (also called γ-glutamyl transpeptidase, henceforth also referred to as “GGT”) is known. It was reported that in a reaction of Val-Gly (valylglycine) and γ-glutamyl-p-nitroanilide as a glutamine donor in the presence of that enzyme, the activity of the enzyme was detected by color development of p-nitroaniline (Non-patent document 1), but generation of γ-Glu-Val-Gly was not confirmed.

GGT of Escherichia coli consists of two subunits, a large subunit and a small subunit, and the ggt gene coding for GGT comprises ORFs (open reading frames) coding for a leader peptide, the large subunit, and the small subunit (Patent document 9). With advance of the transcription and translation of the ggt gene, transfer of the translation product to the periplasm, cleavage of the leader peptide and processing for generating the large subunit and the small subunit occur to generate the mature GGT.

As for researches concerning the structural analysis of GGT, it was reported that the D433N mutation (Non-patent document 2) or the Y444 mutation and G484 mutation (Non-patent document 3) of Escherichia coli GGT impart a novel acylase activity, specifically an ability to hydrolyze glutaryl-7-aminocephalosporanic acid, to GGT, or improve such an ability. Non-patent document 3 (Yamada et al.) specifically describes N411G, N411H, N411Y, Q430I, Q430V, D433A, D433G, D433L, D433N, Y444A, Y444F, Y444G, Y444H, Y444I, Y444L, Y444V, G484A, P460V, L461F, and S462T mutations.

Further, as information concerning the structure around the active center of GGT, it was reported that Y444 in GGT of Escherichia coli (Non-patent document 3) or Y433 in GGT of Helicobacter Pylori (Non-patent document 4) locates in the lid-loop that covers the substrate-binding site.

Furthermore, it was also reported that Arg114 and Arg337 are important for the function of GGT of Escherichia coli (Non-patent document 5), and this reference describes R114K, R114L, R114D, R337K, R337L, and R337D mutations.

Further, it is also reported that most of mutants of Escherichia coli GGT having mutations at Thr407, Asp433, and Met464 in the small subunit lost the activity (Non-patent document 6). This reference describes N411G, N411H, N411Y, Q430I, Q430V, D433A, D433G, D433L, D433N, Y444A, Y444F, Y444G, Y444H, Y444I, Y444L, Y444V, G484A, P460V, L461F, and S462T mutations.

In addition, influences of T391, T392, H393, Q390 and V396 mutations (Non-patent document 7), and R513 and R571 mutations (Non-patent document 8) on processing into the subunits of Escherichia coli GGT or the GGT activity were reported. The former reference describes T391A, T391S, T392A, H393G, Q390A, and V396T mutations, and the latter reference describes R513A, R513G, R571A, and R571G mutations.

However, any mutation or combination of mutations preferred for the γ-glutamylation of Val-Gly is not known.

For the decomposition of Val-Gly, it was reported that PepA hydrolyzes Val-Gly (Non-patent document 9), and PepA- and PepN-deficient Escherichia coli does not decompose Val-Gly (Non-patent document 10), but activity of pepD for decomposing Val-Gly is not known.

PRIOR ART REFERENCES Patent Documents

-   Patent document 1: WO2007/055388 -   Patent document 2: WO2007/055393 -   Patent document 3: WO2008/139945 -   Patent document 4: WO2008/139946 -   Patent document 5: WO2008/139947 -   Patent document 6: WO2009/107660 -   Patent document 7: WO2009/119554 -   Patent document 8: WO2004/011653 -   Patent document 9: Japanese Patent Laid-open (Kokai) No. 02-231085 -   Patent document 10: Japanese Patent Laid-open No. 08-119916

Non-Patent Documents

-   Non-patent document 1: Suzuki, H. et al. (2008) Improvement of the     flavor of amino acids and peptides using bacterial     γ-glutamyltranspeptidase, In Recent Highlights in Flavor Chemistry &     Biology, Ed. by Hofmann, T. et al., pp. 227-232, Deutsche     Forschungsanstalt fur Lebensmittelchemie -   Non-patent document 2: Suzuki, H. et al., Applied and Environmental     Microbiology, 70 (10), 6324-6328, 2004 -   Non-patent document 3: Yamada, C. et al., Appl. Environ. Microbiol.,     74(11):3400-3409, 2008 -   Non-patent document 4: Morrow, A. L. et al., Biochemistry., 20;     46(46):13407-13414, 2007 -   Non-patent document 5: Ong, P. L. et al., Biochem. Biophys. Res.     Commun., 366(2):294-300, 2008 -   Non-patent document 6: Lo, H. F. et al, Indian J. Biochem. Biophys.,     44(4):197-203, 2007 -   Non-patent document 7: Hashimoto, W. et al., J. Biochem.,     118(1):75-80, 1995 -   Non-patent document 8: Hashimoto, W. et al., Biochem. Biophys. Res.     Commun., 189(1):173-178, 1992 -   Non-patent document 9: Gu, Y. Q. et al., Eur. J. Biochem.,     267:1178-1187, 2000 -   Non-patent document 10: Miller, C. G. et al., Journal of     Baccteriol., 135(2):603-611, 1978

DISCLOSURE OF THE INVENTION Object to be Achieved by the Invention

An object of the present invention is to provide a mutant of GGT suitable for the γ-glutamylation of Val-Gly, and a method for producing γ-Glu-Val-Gly or a salt thereof using such a mutant GGT.

Means for Achieving the Object

The inventor of the present invention conducted various researches in order to achieve the aforementioned object, as a result, they found mutations of GGT suitable for the γ-glutamylation of Val-Gly, and accomplished the present invention.

The present invention thus relates to the followings.

(1) A method for producing γ-Glu-Val-Gly comprising the step of reacting Val-Gly with a γ-glutamyl group donor in the presence of a γ-glutamyltransferase, a microorganism containing the enzyme, or a processed product thereof to generate γ-Glu-Val-Gly, wherein:

the γ-glutamyltransferase consists of a large subunit and a small subunit, and the small subunit has the amino acid sequence of the positions 391 to 580 of SEQ ID NO: 2 or the amino acid sequence having a homology of 90% or more to the foregoing amino acid sequence, and has a mutation for one or more residues corresponding to one or more residues selected from the following residues in the amino acid sequence of SEQ ID NO: 2:

N411, T413, Q430, P441, V443, Y444, L446, A453, D472, G484, S498, Q542, D561, S572.

(2) The method as described above, wherein the mutation corresponds to a mutation selected from the following mutations:

N411(Q)

T413(H, N, A)

Q430(M, N)

P441A

V443(E, L, G, N, Q, A)

Y444(D, E, N, A)

L446A

A453S

D472 (I)

G484 (S, A, E)

S498C

Q542H

D561N

S572K.

(3) The method as described above, wherein the mutation is a mutation corresponding to any one of the following mutations:

N411Q, Q430M, Y444A, Y444D, Y444E, G484S, (T413A+Y444E), (T413H+Y444E), (T413N+Y444E), (Q430N+Y444E), (Q430N+Y444D), (Q430N+Y444N), (P441A+Y444E), (V443A+Y444E), (V443E+Y444E), (V443G+Y444E), (V443L+Y444E), (V443N+Y444E), (V443Q+Y444E), (Y444E+L446A), (Y444E+A453S), (Y444E+D472I), (Y444E+G484A), (Y444E+G484S), (Y444E+S498C), (Y444E+Q542H), (Y444E+D561N), (T413N+Y444E+V443A), (T413N+Y444E+A453S), (T413N+Y444E+S498C), (T413N+Y444E+Q542H), (G484S+Y444E+V443A), (G484S+Y444E+Q542H), (Q430N+Y444E+T413N), (T413H+Y444E+G484S), (T413N+Y444E+G484S), (T413N+Y444E+G484S+V443A), (T413N+Y444E+G484S+A453S), (T413N+Y444E+G484S+Q542H), (T413N+Y444E+G484S+S572K) (T413N+Y444E+G484S+Q430N), (T413N+Y444E+G484E+S498C).

(4) The method as described above, wherein the large subunit has the amino acid sequence of the positions 26 to 390 of SEQ ID NO: 2 or the amino acid sequence having a homology of 90% or more to the foregoing amino acid sequence.

(5) The method as described above, wherein the large subunit has a mutation corresponding to any one of the following mutations in the amino acid sequence of SEQ ID NO: 2:

P27H, E38K, L127V, F174A, F1741, F174L, F174M, F174V, F174W, F174Y, T246R, T276N, V301L.

(6) The method as described above, wherein the small subunit has the amino acid sequence of SEQ ID NO: 13 except for the aforementioned mutation.

(7) The method as described above, wherein the small subunit has the amino acid sequence of:

the positions 391 to 580 of SEQ ID NO: 2, the positions 391 to 580 of SEQ ID NO: 3, the positions 392 to 581 of SEQ ID NO: 4, the positions 388 to 577 of SEQ ID NO: 5, the positions 391 to 580 of SEQ ID NO: 6, the positions 391 to 580 of SEQ ID NO: 7, the positions 391 to 580 of SEQ ID NO: 8, the positions 400 to 589 of SEQ ID NO: 9, the positions 391 to 580 of SEQ ID NO: 10, the positions 392 to 581 of SEQ ID NO: 11, or the positions 392 to 581 of SEQ ID NO: 12, or

any one of these amino acid sequences including substitutions, deletions, insertions, additions, or inversions of one or several amino acid residues, except for the aforementioned mutation.

(8) The method as described above, wherein the large subunit has the amino acid sequence of:

the positions 26 to 390 of SEQ ID NO: 2, the positions 26 to 390 of SEQ ID NO: 3, the positions 26 to 391 of SEQ ID NO: 4, the positions 26 to 387 of SEQ ID NO: 5, the positions 25 to 390 of SEQ ID NO: 6, the positions 25 to 390 of SEQ ID NO: 7, the positions 25 to 390 of SEQ ID NO: 8, the positions 33 to 399 of SEQ ID NO: 9, the positions 25 to 390 of SEQ ID NO: 10, the positions 25 to 391 of SEQ ID NO: 11, or the positions 25 to 391 of SEQ ID NO: 12, or

any one of these amino acid sequences including substitutions, deletions, insertions, additions, or inversions of one or several amino acid residues, except for the aforementioned mutation.

(9) The method as described above, wherein the γ-glutamyl group donor is L-glutamine.

(10) The method as described above, wherein the γ-glutamyltransferase, the microorganism containing the enzyme, or the processed product thereof is a microorganism containing the enzyme, or a processed product thereof, and the microorganism is a bacterium belonging to the family Enterobacteriaceae.

(11) The method as described above, wherein the microorganism is an Escherichia bacterium.

(12) The method as described above, wherein the microorganism is Escherichia coli.

(13) The method as described above, wherein the microorganism is deficient in peptidase D.

(14) The method as described above, wherein the reaction is performed in the presence of a metal chelating agent.

(15) A mutant γ-glutamyltransferase consisting of the following large subunit and small subunit:

(A) a large subunit which has the amino acid sequence of the positions 26 to 390 of SEQ ID NO: 2 or the amino acid sequence including substitutions, deletions, insertions, additions, or inversions of one or several amino acid residues, and able to form a complex having the γ-glutamyltransferase activity with any one of the following small subunit;

(B) a small subunit which has the amino acid sequence of the positions 391 to 580 of SEQ ID NO: 2 or the amino acid sequence including substitutions, deletions, insertions, additions, or inversions of one or several amino acid residues, has any one of the following mutations, and able to form a complex having the γ-glutamyltransferase activity with the above large subunit:

Y444D, Y444E, (T413A+Y444E), (T413H+Y444E), (T413N+Y444E), (Q430N+Y444E), (Q430N+Y444D), (Q430N+Y444N), (P441A+Y444E), (V443A+Y444E), (V443E+Y444E), (V443G+Y444E), (V443L+Y444E), (V443N+Y444E), (V443Q+Y444E), (Y444E+L446A), (Y444E+A453S), (Y444E+D472I), (Y444E+G484A), (Y444E+G484S), (Y444E+S498C), (Y444E+Q542H), (Y444E+D561N), (T413N+V443A+Y444E), (T413N+Y444E+A453S), (T413N+Y444E+S498C), (T413N+Y444E+Q542H), (G484S+Y444E+V443A), (G484S+Y444E+Q542H), (Q430N+Y444E+T413N), (T413H+Y444E+G484S), (T413N+Y444E+G484S), (T413N+Y444E+G484S+V443A), (T413N+Y444E+G484S+A453S), (T413N+Y444E+G484S+Q542H), (T413N+Y444E+G484S+S572K), (T413N+Y444E+G484S+Q430N), (T413N+Y444E+G484E+S498C).

(16) The mutant γ-glutamyltransferase as described above, wherein the large subunit has any one of the following mutations:

P27H, E38K, L127V, F174A, F1741, F174L, F174M, F174V, F174W, F174Y, T246R, T276N, V301L.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows alignment of the amino acid sequences of the GGT small subunits of various bacteria. The strains of the bacteria are as follows. The numerals described before and after the amino acid sequences represent positions from the N-terminus of the small subunit. Relations of the amino acid sequences and the amino acid sequences described in Sequence Listing are also shown.

Consensus: consensus sequence (SEQ ID NO: 13)

E. coli: Escherichia coli (SEQ ID NO: 2, 391 to 580)

Sh. flexneri 5 str. 8401: Shigella flexneri 5 str. 8401 (SEQ ID NO: 3, 391 to 580)

Sh. dysenteriae Sd197: Shigella dysenteriae Sd197 (SEQ ID NO: 4, 392 to 581)

Sh. boydii Sb227: Shigella boydii strain Sb227 (SEQ ID NO: 5, 388 to 577)

S. typhimurium ATCC700720: Salmonella enterica typhimurium strain ATCC 700720 (also designated as Salmonella typhimurium LT2, SEQ ID NO: 6, 391 to 580)

S. enterica SC-B67: Salmonella enterica enterica choleraesuis strain SC-B67 (SEQ ID NO: 7, 391 to 580)

S. typhi Ty2: Salmonella enterica typhi strain Ty2 (SEQ ID NO: 8, 391 to 580)

K. pneumoniae ATCC202080: Klebsiella pneumoniae strain ATCC 202080 (SEQ ID NO: 9, 400 to 589)

S. enterica ATCC 9150: Salmonella enterica subsp. enterica serovar Paratyphi A str. ATCC 9150 (SEQ ID NO: 10, 391 to 580)

K. pneumoniae KPN308894: Klebsiella pneumoniae clone KPN 308894 (SEQ ID NO: 11, 392 to 581)

En. cloaceae EBC103795: Enterobacter cloaceae clone

EBC103795 (SEQ ID NO: 12, 392 to 581)

FIG. 2 shows continuation of FIG. 1.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

Hereafter, the present invention will be explained in detail.

The method for producing γ-Glu-Val-Gly or a salt thereof of the present invention comprises the step of reacting Val-Gly or a salt thereof with a γ-glutamyl group donor in the presence of GGT, a microorganism containing the enzyme, or a processed product thereof to generate γ-Glu-Val-Gly or a salt thereof. The method of the present invention is characterized in that GGT consists of a large subunit and a small subunit, and at least the small subunit has a specific mutation. Hereafter, GGT having such a specific mutation and the method for producing γ-Glu-Val-Gly or a salt thereof using it will be explained.

In this specification, amino acids are L-amino acids, unless especially mentioned.

<1> Mutant GGT

GGT having the aforementioned specific mutation (also referred to as “mutant GGT”) can be obtained by modifying a ggt gene coding for GGT not having the specific mutation, so that the encoded GGT has the specific mutation, and expressing the obtained modified ggt gene. GGT not having the aforementioned specific mutation may be referred to as a wild-type GGT, and a ggt gene coding for the wild-type GGT may be referred to as a wild-type ggt gene. The wild-type GGT may have other mutations, so long as it does not have the specific mutation. The specific mutation will be explained later.

Examples of the wild-type GGT include GGT encoded by the ggt gene of Escherichia coli and homologues thereof, for example, GGT of Escherichia coli, and GGTs of other microorganisms, especially those of which small subunit has a similar structure.

The nucleotide sequence of the ggt gene of the Escherichia coli K-12 strain is described in Japanese Patent Laid-open No. 02-231085. Further, the nucleotide sequence of the ggt gene of the Escherichia coli K-12 W3110 strain is registered in the database as 4053592.4055334 of GenBank accession AP009048. The nucleotide sequence of this ggt gene is shown in SEQ ID NO: 1. Further, the amino acid sequence encoded by this nucleotide sequence is shown in SEQ ID NO: 2. In SEQ ID NO: 2, the positions 1 to 25 correspond to the leader peptide, the positions 26 to 390 correspond to the large subunit, and the positions 391 to 580 correspond to the small subunit.

As GGT homologues homologous to GGT of Escherichia coli, those containing a small subunit having an amino acid sequence showing a homology of 90% or more to the site corresponding to the small subunit in the amino acid sequence shown in SEQ ID NO: 2 (positions 391 to 580) are preferred. As the GGT homologues, those containing a large subunit having an amino acid sequence showing a homology of 90% or more to the site corresponding to the large subunit in the amino acid sequence shown in SEQ ID NO: 2 (positions 26 to 390) are preferred. Specific examples include GGTs of bacteria belonging to the family Enterobacteriaceae. Although the bacteria belonging to the family Enterobacteriaceae are not particularly limited, they include bacteria belonging to the genera of Escherichia, Enterobacter, Erwinia, Klebsiella, Pantoea, Photorhabdus, Providencia, Salmonella, Serratia, Shigella, Morganella, Yersinia, and so forth. In particular, bacteria classified into the family Enterobacteriaceae according to the taxonomy used in the NCBI (National Center for Biotechnology Information) database (http(colon)//www(dot)ncbi(dot)nlm(dot)nih(dot)gov/Taxonomy/Browser/wwwtax(dot)cgi?id=91347) are preferred. Specific examples include, for example, Shigella flexneri, Shigella dysenteriae, Shigella boydii, Salmonella typhimurium, Klebsiella pneumoniae, Salmonella enterica, Enterobacter cloacae, and so forth. The amino acid sequences of GGTs of Shigella flexneri 5 str. 8401 (GenBank accession ABF05491), Shigella dysenteriae Sd197 (GenBank accession ABB63568), Shigella boydii Sb227 (GenBank accession ABB67930), Salmonella enterica typhimurium strain ATCC 700720 (also designated as Salmonella typhimurium LT2, GenBank accession AAL22411), Salmonella enterica enterica choleraesuis strain SC-B67 (GenBank accession AAX67386), Salmonella enterica typhi strain Ty2 (GenBank accession AAO71440), Klebsiella pneumoniae ATCC 202080 (U.S. Pat. No. 6,610,836, SEQ ID NO: 10810), Salmonella enterica subsp. enterica serovar Paratyphi A str. ATCC 9150 (GenBank accession AAV79214), Klebsiella pneumoniae clone KPN308894 (WO02/077183, SEQ ID NO: 60310), and Enterobacter cloacae clone EBC103795 (WO02/077183, SEQ ID NO: 56162) are shown in SEQ ID NOS: 3 to 12.

The positions of the leader peptide, the large subunit, and the small subunit in each GGT are shown in Table 1. Further, alignment of the amino acid sequences of the small subunits of those GGTs is shown in FIGS. 1 and 2. The consensus sequence of the small subunit amino acid sequences is shown in FIGS. 1 and 2 and SEQ ID NO: 13. Further, identities of the small subunits of those bacteria and the small subunit of Escherichia coli are shown in Table 1. The amino acid codes used in FIGS. 1 and 2 are as follows. Specific examples of X include the amino acid residues locating at positions corresponding to the positions of X in the GGT small subunits of the bacteria described in the alignment shown in FIGS. 1 and 2. For example, X at the position 4 of SEQ ID NO: 13 is Tyr or Phe.

A: Ala, C: Cys, D: Asp, E: Glu, F: Phe, G: Gly, H: His, I: Ile, K: Lys, L: Leu, M: Met, N: Asn, P: Pro, Q: Gln, R: Arg, S: Ser, T: Thr, V: Val, W: Trp, X: Xaa, Y: Tyr

TABLE 1 Identity SEQ of small ID Leader Large Small subunit Bacterium NO peptide subunit subunit (%) Escherichia coli 2 1-25 26-390 391-580 Shigella flexneri 5 str. 3 1-25 26-390 391-580 99 8401 Shigella dysenteriae Sd197 4 1-25 26-391 392-581 99 Shigella boydii strain 5 1-25 26-387 388-577 99 Sb227 Salmonella enterica 6 1-24 25-390 391-580 95 typhimurium strain ATCC 700720 Salmonella enterica 7 1-24 25-390 391-580 95 enterica choleraesuis strain SC-B67 Salmonella enterica typhi 8 1-24 25-390 391-580 95 strain Ty2 Klebsiella pneumoniae 9 1-32 33-399 400-589 95 strain ATCC 202080 Salmonella enterica subsp. 10 1-24 25-390 391-580 94 enterica serovar Paratyphi A str. ATCC 9150 Klebsiella pneumoniae 11 1-24 25-391 392-581 95 clone KPN308894 Enterobacter cloaceae 12 1-24 25-391 392-581 93 clone EBC103795

Preferred examples of the wild-type GGT are those having the amino acid sequence of SEQ ID NO: 13 in the small subunit.

The mutant GGT of the present invention has one or more mutations at one or more residues selected from the following residues in the small subunit:

N411, T413, Q430, P441, V443, Y444, L446, A453, D472, G484, S498, Q542, D561, 5572.

In the above indications, the letters on the left side of the numerals represent type of amino acid residue, and the numerals represent position in GGT. The position of amino acid residue is represented as the position on the amino acid sequence of the GGT precursor (protein consisting of the leader peptide, the large subunit, and the small subunit connected in this order) encoded by ORFs of the ggt gene. For example, the amino acid residue of the N-terminus of the GGT small subunit of Escherichia coli corresponds to the position 26 of SEQ ID NO: 2. The term amino acid sequence of GGT may henceforth mean the amino acid sequence of the GGT precursor, unless especially indicated.

The amino acid residue existing at the position of the mutation after the substitution may be any amino acid residue so long as it is an amino acid residue other than the original amino acid residue, and specific examples of the mutation include those selected from the following mutations:

N411Q

T413(H, N, A)

Q430(M, N)

P441A

V443(E, L, G, N, Q, A)

Y444(D, E, N, A)

L446A

A453S

D472I

G484(S, A, E)

S498C

Q542H

D561N

S572K.

The meanings of the letters representing the type of amino acid in the indications of the mutations are the same as those described above. The numerals represent the positions of the mutation. The letters on the left side of the numerals represent the amino acid residues existing in the wild-type, and the letters on the right side of the numerals represent the amino acid residue existing after the mutation. For example, “N411Q” means a substitution of Gln residue for the Asn residue at the position 411. Further, the letters in the parentheses on the right side of the numerals collectively represent amino acid residues existing after the mutation. For example, T413(H, N, A) means substitution of His, Asn or Ala residue for the Thr residue at the position 413.

More specific examples of the mutant GGT of the present invention include those having any one of the following mutations in the small subunit. The collective indications of two or more mutations using the symbol “+” means a double mutation or a more multiple mutation. For example, (N411Q+Q430N) means that the mutant GGT simultaneously has the N411Q mutation and the Q430N mutation.

N411Q, T413H, T413N, T413A, Q430M, Q430N, P441A, V443E, V443L, V443G, V443N, V443Q, V443A, Y444D, Y444E, Y444N, Y444A, L446A, A453S, D472I, G484S, G484A, G484E, S498C, Q542H, D561N, S572K,

(N411Q+Q430N),

(N411Q+S572K)

(T413N+G484E)

(T413N+G484S)

(T413A+Y444E)

(T413H+Y444E)

(T413N+Y444E)

(Q430N+Y444E)

(Q430N+Y444D)

(Q430N+Y444N)

(P441A+Y444E)

(V443A+Y444E)

(V443E+Y444E)

(V443G+Y444E)

(V443L+Y444E)

(V443N+Y444E)

(V443Q+Y444E)

(V443A+Y444E)

(Y444E+L446A)

(Y444E+A453S)

(Y444E+D472I)

(Y444E+G484A)

(Y444E+G484S)

(Y444E+Q542H)

(Y444E+D561N)

(T413N+Y444E+V443A)

(T413N+Y444E+A453S)

(T413N+Y444E+S498C)

(T413N+Y444E+Q542H)

(G484S+Y444E+T276N)

(G484S+Y444E+V443A)

(G484S+Y444E+Q542H)

(Q430N+Y444E+T413N)

(T413H+Y444E+G484S)

(T413N+Y444E+G484S)

(T413N+Y444E+G484S+V443A)

(T413N+Y444E+G484S+A453S)

(T413N+Y444E+G484S+Q542H)

(T413N+Y444E+G484S+S572K)

(T413N+Y444E+G484S+Q430N)

(T413N+Y444E+G484E+S498C)

As for the aforementioned mutations, the positions 413, 472, and 572 are novel mutation sites, and N411Q, Q430M, Q430N, Q430P, Q430S, Q430Y, Y444D, Y444E, D472S, and G484S are novel mutations.

Among the aforementioned mutations, preferred are those with which generation of γ-Glu-Val-Gly was confirmed in Example 5. Further, those especially preferred for the method of the present invention are those with which γ-Glu-Val-Gly generation amount of 40 mM or more concerning a single mutation, or 60 mM or more concerning a complex mutation was obtained in Example 5. For example, the enzyme having N411Q, Q430M, Y444A, Y444D, Y444E, G484S, a double mutation, triple mutation, or quadruple mutation of the foregoing mutations is preferred.

Further, from another aspect, the Y444E mutation or a complex mutation of the Y444E mutation and one or more other mutations among the aforementioned mutations is also preferred.

Furthermore, as the mutant γ-glutamyltransferase of the present invention, especially preferred are mutant enzymes showing a higher activity compared with the known mutant γ-glutamyltransferase having the Y444A mutation (Glu-Val-Gly production amount observed in Example 5 is 52.5 mM), such as the enzymes having Y444D, Y444E, a double mutation, triple mutation, or quadruple mutation of the aforementioned mutations.

As for GGTs of microorganisms other than Escherichia coli, the positions of the mutations thereof corresponding to the mutations on the amino acid sequence of GGT of Escherichia coli are represented by the corresponding positions in GGT of Escherichia coli determined in alignment of the amino acid sequences of the GGTs of the other microorganisms and the amino acid sequence of GGT of Escherichia coli. For example, an amino acid residue of a position 100 in an amino acid sequence of GGT of a certain microorganism corresponds to the position 101 of the amino acid sequence of GGT of Escherichia coli in the alignment, that amino acid residue of the position 100 is regarded as the amino acid residue of the position 101. In the present invention, a residue corresponding to a specific residue in SEQ ID NO: 2 means a residue corresponding to the specific amino acid residue in the amino acid sequence of SEQ ID NO: 2 in the alignment of the amino acid sequence of SEQ ID NO: 2 and an objective sequence, as described above. Similarly, a mutation corresponding to a specific mutation in SEQ ID NO: 2 is a mutation at a residue corresponding to a residue of the specific mutation in the amino acid sequence of SEQ ID NO: 2 in the alignment of the amino acid sequence of SEQ ID NO: 2 and an objective sequence, as described above.

As a means for performing the alignment, known gene analysis software can be used. Specific examples of such software include DNASIS produced by Hitachi Solutions, GENETYX produced by Genetyx, and so forth (Elizabeth C. Tyler et al., Computers and Biomedical Research, 24(1), 72-96, 1991; Barton G J et al., Journal of Molecular Biology, 198(2), 327-37, 1987).

The positions of the mutations do not necessarily represent the absolute positions from the N-terminus in the amino acid sequences of mutant GGTs, and represent relative positions with respect to the amino acid sequence shown in SEQ ID NO: 2. For example, if one amino acid residue of GGT having the amino acid sequence shown in SEQ ID NO: 2 is deleted at a position on the N-terminus side with respect to a position n, this position n becomes an (n−1) position from the N-terminus. However, even in such a case, the amino acid residue of this position is regarded as the amino acid residue of the position n. An absolute position of an amino acid substitution can be determined on the basis of alignment of an amino acid sequence of an objective GGT and the amino acid sequence of SEQ ID NO: 2. The method for performing the alignment for this case is the same as the method described above.

Although the mutant GGT of the present invention may not contain a mutation in the large subunit, it may have any of the mutations shown below:

P27H, E38K, L127V, F174A, F1741, F174L, F174M, F174V, F174W, F174Y, T246R, T276N, V301L.

Examples of the combination of the mutations of the small subunit and the large subunit include the following combinations:

(E38K+Y444E)

(F174A+Y444E)

(F1741+Y444E)

(F174L+Y444E)

(F174M+Y444E)

(F174V+Y444E)

(F174W+Y444E)

(F174Y+Y444E)

(T246R+Y444E)

(V301L+Y444E)

(T413N+Y444E+P27H)

(T413N+Y444E+E38K)

(T413N+Y444E+L127V)

(T413N+Y444E+T276N)

(G484S+Y444E+T276N)

(G484S+Y444E+P27H)

Furthermore, the mutant GGT of the present invention may be a conservative variant of the proteins having the aforementioned amino acid sequences, i.e., a homologue, an artificially modified protein or the like of the proteins concerning the amino acid sequence thereof, such as any of the amino acid sequences of SEQ ID NOS: 2 to 12, so long as the GGT activity is not degraded. That is, the mutant GGT of the present invention may have any of the aforementioned amino acid sequences including, in addition to the aforementioned specific mutations, substitutions, deletions, insertions, additions, or inversions of one or several amino acid residues. Although the number meant by the term “one or several” can differ depending on the positions of amino acid residues in the three-dimensional structure or the types of amino acid residues of the protein, specifically, it is preferably 1 to 20, more preferably 1 to 10, still more preferably 1 to 5. The conservative mutation is typically a conservative substitution. The conservative substitution is a mutation wherein substitution takes place mutually among Phe, Trp, and Tyr, if the substitution site is an aromatic amino acid; among Leu, Ile and Val, if the substitution site is a hydrophobic amino acid; between Gln and Asn, if the substitution site is a polar amino acid; among Lys, Arg and His, if the substitution site is a basic amino acid; between Asp and Glu, if the substitution site is an acidic amino acid; and between Ser and Thr, if the substitution site is an amino acid having a hydroxyl group. Substitutions considered conservative substitutions include, specifically, substitution of Ser or Thr for Ala, substitution of Gln, His or Lys for Arg, substitution of Glu, Gln, Lys, His or Asp for Asn, substitution of Asn, Glu or Gln for Asp, substitution of Ser or Ala for Cys, substitution of Asn, Glu, Lys, His, Asp or Arg for Gln, substitution of Gly, Asn, Gln, Lys or Asp for Glu, substitution of Pro for Gly, substitution of Asn, Lys, Gln, Arg or Tyr for His, substitution of Leu, Met, Val or Phe for Ile, substitution of Ile, Met, Val or Phe for Leu, substitution of Asn, Glu, Gln, His or Arg for Lys, substitution of Ile, Leu, Val or Phe for Met, substitution of Trp, Tyr, Met, Ile or Leu for Phe, substitution of Thr or Ala for Ser, substitution of Ser or Ala for Thr, substitution of Phe or Tyr for Trp, substitution of His, Phe or Trp for Tyr, and substitution of Met, Ile or Leu for Val. The aforementioned amino acid substitutions, deletions, insertions, additions, inversions or the like can be a result of a naturally-occurring mutation due to an individual difference, difference of species, or the like of a microorganism from which the gene is derived (mutant or variant).

Further, GGT having such a conservative mutation as described above can be a protein showing a homology of, for example, 80% or more, preferably 90% or more, more preferably 95% or more, still more preferably 97% or more, particularly preferably 99% or more, to any of the amino acid sequences of SEQ ID NOS: 2 to 12, and having the GGT activity.

In this specification, “homology” can mean “identity”.

Further, GGT may be a protein encoded by a DNA that is able to hybridize with a prove having a nucleotide sequence complementary to the nucleotide sequence of SEQ ID NO: 1, or a probe that can be prepared from the complementary sequence under stringent conditions, and having the GGT activity. The “stringent conditions” refer to conditions under which a so-called specific hybrid is formed, and a non-specific hybrid is not formed. Examples of the stringent conditions include those under which highly homologous DNAs hybridize to each other, for example, DNAs not less than 80% homologous, preferably not less than 90% homologous, more preferably not less than 95% homologous, still more preferably not less than 97% homologous, particularly preferably not less than 99% homologous, hybridize to each other, and DNAs less homologous than the above do not hybridize to each other, for example, conditions of hybridization at 42° C. and washing with a buffer containing 1×SSC and 0.1% SDS at 42° C., more preferably conditions of hybridization at 65° C. and washing with a buffer containing 0.1×SSC and 0.1% SDS at 65° C. The factors affecting the stringency for hybridization include various factors other than the aforementioned temperature conditions, and those skilled in the art can realize a stringency corresponding to the stringency exemplified above by using an appropriate combination of the various factors.

The probe used for the hybridization may be a part of a complementary sequence of the ggt gene. Such a probe can be produced by PCR using oligonucleotides synthesized on the basis of a known gene sequence as primers and a DNA fragment containing the nucleotide sequence as a template. For example, when a DNA fragment having a length of about 300 bp is used as the probe, the washing conditions of the hybridization can be, for example, 50° C., 2×SSC and 0.1% SDS.

The mutant GGT of the present invention can be produced by inserting a mutant ggt gene coding for it into an appropriate vector, and introducing the obtained recombinant vector into an appropriate host to allow expression thereof. Further, a microorganism containing the mutant GGT used for the method for producing γ-Glu-Val-Gly described later can be obtained by introducing the recombinant vector into an appropriate host microorganism.

The ggt gene having the specific mutation can be obtained by, for example, modifying a nucleotide sequence of a wild-type ggt gene, for example, a ggt gene coding for any of the amino acid sequences of SEQ ID NOS: 2 to 12, by the site-directed mutagenesis method, so that the amino acid residue of the specific position of the encoded GGT is replaced with another amino acid residue.

Examples of the site-directed mutagenesis method for introducing an objective mutation at an intended site of DNA include, for example, a method using PCR (Higuchi, R., 61, in PCR technology, Erlich, H. A. Eds., Stockton Press, 1989; Carter P., Meth. In Enzymol., 154, 382, 1987), as well as a method of using a phage (Kramer, W. and Frits, H. J., Methods in Enzymology, 154, 350, 1987; Kunkel, T. A. et al., Methods in Enzymology, 154, 367, 1987), and so forth.

The vector into which a mutant ggt gene is incorporated is not particularly limited so long as a vector that can replicate in the host is chosen. When Escherichia coli is used as the host, examples of such a vector include plasmids that can autonomously replicate in this bacterium. For example, pUC19, pET, pGEMEX, pGEM-T and so forth can be used. Preferred examples of the host include Escherichia coli strains. However, other than these, any of microorganisms in which a replication origin and a mutant ggt gene of a constructed recombinant DNA can function, a recombinant DNA can be expressed, and the mutant ggt gene can be expressed can be used as the host. As the host, for example, Gram-negative bacteria including Escherichia bacteria such as Escherichia coli, Enterobacter bacteria, Pantoea bacteria, and so forth, and Gram-positive bacteria including Bacillus bacteria, Corynebacterium bacteria, and so forth can be used. For example, Bacillus subtilis is known to secrete produced GGT out of cells (Xu et al., Journal of Bacteriology, Vol. 178, No. 14, 1996), and a mutant GGT may be secreted out of cells. In addition, an objective mutant ggt gene may be expressed by using cells of yeast, mold, or the like.

Examples of transformation methods include treating recipient cells with calcium chloride to increase permeability for DNA, which has been reported for Escherichia coli K-12 (Mandel, M. and Higa, A., J. Mol. Biol., 53:159-162, 1970), preparing competent cells from cells which are at the growth phase, followed by transformation with DNA, which has been reported for Bacillus subtilis (Duncan, C. H., Wilson, G. A. and Young, F. E., Gene, 1:153-167, 1977), and so forth. Alternatively, a method of making DNA-recipient cells into protoplasts or spheroplasts, which can easily take up recombinant DNA, followed by introducing a recombinant DNA into the cells, which is known to be applicable to Bacillus subtilis, actinomycetes and yeasts (Chang, S, and Choen, S. N., Mol. Gen. Genet., 168:111-115, 1979; Bibb, M. J., Ward, J. M. and Hopwood, O. A., Nature, 274:398-400, 1978; Hinnen, A., Hicks, J. B. and Fink, G. R., Proc. Natl. Sci., USA, 75:1929-1933, 1978) can also be employed. In addition, transformation of microorganisms can also be performed by the electroporation method (Japanese Patent Laid-open No. 2-207791).

The promoter for expressing the mutant ggt gene may be a promoter inherent to the ggt gene, or a promoter of another gene. Examples of promoters of other genes include rpoH promoter, lac promoter, trp promoter, trc promoter, tac promoter, PR promoter and PL promoter of lambda phage, tet promoter, and so forth.

Further, as a vector into which the ggt gene is inserted, an expression vector containing a promoter suitable for gene expression may also be used.

A transformant introduced with the recombinant DNA containing the mutant ggt gene obtained as described above can be cultured in an appropriate medium containing a carbon source, a nitrogen source, inorganic ions, and organic nutrients if needed to allow expression of the mutant GGT.

When a microorganism that expresses a mutant ggt gene is used for producing γ-Glu-Val-Gly, the microorganism is preferably deficient in the peptidase D (PepD). The term “deficient in PepD” means that the microorganism is completely deficient in PepD, or the microorganism has a reduced amount or activity of PepD compared with a wild-type strain. The inventors of the present invention found that PepD is deeply involved in the decomposition of Val-Gly in Escherichia coli. By making a microorganism that expresses a mutant GGT deficient in PepD, the generation amount of γ-Glu-Val-Gly, which is generated from Val-Gly as a substrate, can be increased.

The microorganism can be made deficient in PepD by, for example, reducing expression of the pepD gene coding for PepD. The expression of the pepD gene can be reduced by modifying the pepD gene on a chromosome so that a wild-type RNA or wild-type protein is not expressed, for example, by disrupting the pepD gene. As methods for such gene disruption, there are the method utilizing a linear DNA such as the method called “Red-driven integration” (Datsenko, K. A. and Wanner, B. L., Proc. Natl. Acad. Sci. USA, 97:6640-6645, 2000), and the method based on the combination of the Red-driven integration method and the λ phage excision system (Cho, E. H., Gumport, R. I., Gardner, J. F., J. Bacteriol., 184:5200-5203, 2002) (refer to WO2005/010175), the methods utilizing a plasmid having a temperature sensitive replication origin, a plasmid capable of conjugative transfer, a suicide vector not having a replication origin in a host, and so forth (U.S. Pat. No. 6,303,383, or Japanese Patent Laid-open No. 05-007491).

The nucleotide sequence of the pepD gene of Escherichia coli and the amino acid sequence encoded by this gene are shown in SEQ ID NOS: 14 and 15, respectively. Further, the nucleotide sequence of pepA, pepB, pepE, and pepN genes, which are other peptidase genes of Escherichia coli, are shown in SEQ ID NOS: 16, 18, 20, and 22, respectively. Further, the amino acid sequences encoded by these genes are shown in SEQ ID NOS: 17, 19, 21, and 23, respectively. So long as the microorganism that expresses a mutant ggt gene is deficient in PepD, it may be deficient in another arbitrary peptidase.

<2> Method for Producing γ-Glu-Val-Gly or Salt Thereof.

By reacting Val-Gly or a salt thereof with a γ-glutamyl group donor in the presence of the mutant GGT obtained as described above, a microorganism containing the mutant GGT, or a processed product thereof to generate γ-Glu-Val-Gly or a salt thereof, γ-Glu-Val-Gly or a salt thereof can be produced.

A microorganism containing the mutant GGT can be produced by culturing a microorganism into which the mutant ggt gene has been introduced in an expressible form under conditions enabling expression of the gene to allow growth of cells. The medium used for the culture is not particularly limited so long as the objective microorganism can grow in it, and there can be used a conventional medium containing a carbon source, a nitrogen source, a sulfur source, inorganic ions, and other organic components as required.

As the carbon source, saccharides such as glucose, fructose, sucrose, glycerol, ethanol, molasses and starch hydrolysate, and organic acids such as fumaric acid, citric acid and succinic acid can be used.

As the nitrogen source, inorganic ammonium salts such as ammonium sulfate, ammonium chloride and ammonium phosphate, organic nitrogen such as soybean hydrolysate, ammonia gas, aqueous ammonia and so forth can be used.

Examples of the sulfur source include inorganic sulfur compounds, such as sulfates, sulfites, sulfides, hyposulfites and thiosulfates.

As organic trace amount nutrients, it is desirable to add required substances such as vitamin B₁, yeast extract and so forth in appropriate amounts. Other than these, potassium phosphate, magnesium sulfate, iron ions, manganese ions and so forth are added in small amounts.

The culture conditions can be appropriately chosen according to the microorganism to be used. The culture is preferably performed at a culture temperature of, for example, 20 to 45° C., preferably 24 to 45° C. The culture is preferably performed as aeration culture at an oxygen concentration of 5 to 50%, desirably about 10%, with respect to the saturated concentration. Further, pH during the culture is preferably 5 to 9. For adjusting pH, inorganic or organic acidic or alkaline substances, such as calcium carbonate, ammonia gas, and aqueous ammonia, can be used.

By culturing the microorganism preferably for about 10 to 120 hours under such conditions as described above, the mutant GGT is accumulated in the periplasm of cells.

In addition, by appropriately choosing the host to be used and designing the ggt gene, it is also possible to accumulate GGT in cells or produce GGT with allowing secretion thereof out of cells.

The mutant GGT may be used in a state of being contained in cells, or may be used as a crude enzyme fraction extracted from the cells or a purified enzyme. The mutant GGT can be extracted by the same method as those for conventional extraction of a periplasmic enzyme, for example, osmotic shock method, freezing and thawing method, and so forth. Further, the mutant GGT can be purified by an appropriate combination of methods usually used for purification of enzyme, such as ammonium sulfate fractionation, ion exchange chromatography, hydrophobic chromatography, affinity chromatography, gel filtration chromatography, and electrofocusing. When GGT is produced and secreted out of cells, the mutant GGT collected from the medium can be used.

The processed product of the microorganism containing mutant GGT is not particularly limited so long as it contains the mutant GGT in a state that the mutant GGT can function, and examples include disrupted cells, cell extract, partially purified products thereof, purified enzyme, and so forth, as well as cells immobilized with acrylamide, carrageenan, or the like, immobilized enzymes comprising the mutant GGT immobilized on a solid phase such as resin, and so forth.

In the presence of the mutant GGT, a microorganism containing the mutant GGT or a processed product thereof obtained as described above, Val-Gly and a γ-glutamyl group donor are reacted.

Val-Gly or a salt thereof can be produced by a chemical synthesis method using formyl-L-valine and glycine ethyl ester as the starting materials (Journal of the American Chemical Society, 80, 1154-1158, 1958). Alternatively, it is also possible to use a chemical synthesis method using N-carboxyanhydride of valine (valine-NCA) and glycine as the starting materials (Canadian Journal of Chemistry, 51 (8), 1284-87, 1973). Further, other methods known as peptide synthesis methods can also be used (“Fundamentals and Experiments of Peptide Synthesis”, Maruzen Co., Ltd., 1985).

Further, when Val-Gly to be used is in the form of a salt, it may be any salt so long as a chemically acceptable salt is used. Specific examples of the “chemically acceptable salt” include, for acidic groups such as carboxyl group, ammonium salt, salts with alkali metals such as sodium and potassium, salts with alkaline earth metals such as calcium and magnesium, aluminum salts, zinc salts, salts with organic amines such as triethylamine, ethanolamine, morpholine, pyrrolidine, piperidine, piperazine, and dicyclohexylamine, salts with basic amino acids such as arginine and lysine, and for basic groups, salts with inorganic acids such as hydrochloric acid, sulfuric acid, phosphoric acid, nitric acid, and hydrobromic acid, salts with organic carboxylic acids such as acetic acid, citric acid, benzoic acid, maleic acid, fumaric acid, tartaric acid, succinic acid, tannic acid, butyric acid, hibenzic acid, pamoic acid, enanthic acid, decanoic acid, teoclic acid, salicylic acid, lactic acid, oxalic acid, mandelic acid, and malic acid, and salts with organic sulfonic acid such as methanesulfonic acid, benzenesulfonic acid, and p-toluenesulfonic acid.

Furthermore, when γ-Glu-Val-Gl obtained by the method of the present invention is in the form of a salt, it may be a chemically and pharmaceutically acceptable edible salt, and examples include, for acidic groups such as carboxyl group, ammonium salts, salts with alkali metals such as sodium and potassium, salts with alkaline earth metals such as calcium and magnesium, aluminum salts, zinc salts, salts with organic amines such as triethylamine, ethanolamine, morpholine, pyrrolidine, piperidine, piperazine, and dicyclohexylamine, salts with basic amino acids such as arginine and lysine, and for basic groups, salts with inorganic acids such as hydrochloric acid, sulfuric acid, phosphoric acid, nitric acid, and hydrobromic acid, salts with organic carboxylic acids such as acetic acid, citric acid, benzoic acid, maleic acid, fumaric acid, tartaric acid, succinic acid, tannic acid, butyric acid, hibenzic acid, pamoic acid, enanthic acid, decanoic acid, teoclic acid, salicylic acid, lactic acid, oxalic acid, mandelic acid, and malic acid, and salts with organic sulfonic acid such as methanesulfonic acid, benzenesulfonic acid, and p-toluenesulfonic acid. When it is used for foods, it is sufficient that it is an edible salt.

Peptide production using an enzymatic reaction can be performed by using any of the following methods reported as methods for producing a peptide, namely, a condensation reaction using an N-protected and C-non-protected carboxyl component and an N-non-protected and C-protected amine component (reaction 1), a substitution reaction using an N-protected and C-protected carboxyl component and an N-non-protected and C-protected amine component (reaction 2), a substitution reaction using an N-non-protected and C-protected carboxyl component and an N-non-protected and C-protected amine component (reaction 3), a substitution reaction using an N-non-protected and C-protected carboxyl component and an N-non-protected and C-non-protected amine component (reaction 4), or a transfer reaction using an N-non-protected and C-protected carboxyl component and an N-non-protected and C-non-protected amine component (reaction 5), and purifying Val-Gly or a salt thereof from the reaction product.

Examples of the reaction 1 include, for example, the method for producing Z-aspartylphenylalanine methyl ester from Z-aspartic acid and phenylalanine methyl ester (Japanese Patent Laid-open No. 53-92729), examples of the reaction 2 include, for example, the method for producing acetylphenylalanylleucinamide from acetylphenylalanine ethyl ester and leucinamide (Biochemical J., 163, 531, 1977), examples of the reaction 3 include, for example, the method for producing arginylleucinamide from arginine ethyl ester and leucinamide (WO90/01555), examples of the reaction 4 include, for example, the method for producing tyrosylalanine from tyrosine ethyl ester and alanine (EP 278787 A1, EP 359399 B1), and examples of the reaction 5 include, for example, the method for producing alanylglutamine from alanine methyl ester and glutamine (W2004/011653). It is possible to apply the above reactions to the production of Val-Gly or a salt thereof. In such a case, by reacting valine having an esterified or amidated carboxyl group and glycine having free amino group in the presence of a peptide-producing enzyme, and purifying Val-Gly from the reaction product, Val-Gly can be produced.

The γ-glutamyl group donor can be chosen from γ-glutamyl compounds. Examples include, for example, glutamine, glutamic acid γ-alkyl esters such as glutamic acid γ-methyl ester, salts thereof, and so forth. Among these, glutamine and a salt thereof are preferred. This salt may also be such a chemically acceptable salt as explained above, and the definition thereof is the same as described above.

The reaction of Val-Gly or a salt thereof and the γ-glutamyl group donor may be performed by the batch method or the column method. When the batch method is used, Val-Gly or a salt thereof, the γ-glutamyl group donor and the mutant GGT, a microorganism containing the mutant GGT, or a processed product thereof can be mixed in a reaction mixture contained in a reaction vessel. The reaction may be performed as a standing reaction, or with stirring. When the column method is used, a reaction mixture containing Val-Gly or a salt thereof and the γ-glutamyl group donor can be passed thorough a column filled with such immobilized cells or immobilized enzyme as described above.

The reaction mixture preferably consists of water or a buffer containing Val-Gly or a salt thereof and the γ-glutamyl group donor, and preferably has pH of 6.0 to 10.0, more preferably 6.5 to 9.0.

The reaction time or the flow rate of the reaction mixture can be appropriately determined according to the concentrations of the substrates, amount of the mutant GGT with respect to the substrates, and so forth. Specifically, for example, the amount of the enzyme to be added can be determined by measuring the enzyme activity under a certain condition, and determining the amount on the basis of the measured activity value. For example, the enzyme activity can be measured by using an appropriate amount of enzyme with a composition of the reaction mixture of 0.1 M glutamine, 0.1 M Val-Gly, and 0.1 M potassium phosphate (pH 7.6), a reaction temperature of 37° C., and a reaction time of 1 to 10 minutes. For example, when the amount of the enzyme which produces 1 μmol of γ-Glu-Val-Gly in 1 minute under the aforementioned conditions is defined to be 1 U, the reaction can be performed with substrate concentrations of 1 to 2000 mM glutamine as the γ-glutamyl group donor and 1 to 2000 mM Val-Gly, as well as an enzyme concentration of 0.1 to 100 U/ml.

The reaction temperature is usually 15 to 50° C., preferably 15 to 45° C., more preferably 20 to 40° C.

Although the molar ratio of Val-Gly or a salt thereof and the γ-glutamyl group donor in the reaction mixture may vary depending on the type of the γ-glutamyl group donor used for the reaction, the molar ratio of Val-Gly:γ-glutamyl group donor is usually preferably 1:0.1 to 1:10. The concentrations of Val-Gly and the γ-glutamyl group donor in the reaction mixture are usually 1 to 2000 mM, preferably 100 to 2000 mM, more preferably 100 to 1000 mM.

The amount of the mutant GGT to the substrates is usually 0.01 to 1000 U, preferably 0.1 to 500 U, more preferably 0.1 to 100 U, with respect to 1 mmol of the substrates.

When a microorganism containing the mutant GGT or a processed product thereof is used, if a peptidase, especially PepD, is contained, Val-Gly as the substrate and/or γ-Glu-Val-Gly as the product may be easily decomposed. Therefore, it is preferable to use a PepD-deficient strain as the microorganism. Alternatively, the peptidase activity can also be suppressed by adding a metal chelating agent which chelates metal ions required for the enzyme activity of peptidase, such as Co²⁺, Mn²⁺, and Fe²⁺, to the reaction mixture. Examples of the metal chelating agent include EDTA and so forth. The concentration of the metal chelating agent in the reaction mixture is usually 0.01 to 500 mM, preferably 0.01 to 100 mM, more preferably 0.1 to 10 mM. When a purified mutant GGT or purified mutant GGT not containing the peptidase activity is used, the metal chelating agent is unnecessary, but it may be contained.

As described above, γ-Glu-Val-Gly is produced in the reaction mixture. γ-Glu-Val-Gly or a salt thereof can be collected from the reaction mixture by, for example, various chromatography techniques such as ion exchange chromatography, reversed phase high performance liquid chromatography, and affinity chromatography, crystallization and recrystallization from a solution, and so forth.

EXAMPLES

Hereafter, the present invention will be still more specifically explained with reference to examples.

Example 1 Construction of GGT Expression Plasmid

A GGT expression plasmid was constructed by inserting the ggt gene of Escherichia coli into an expression plasmid pSF12_Sm_Aet containing the rpoH promoter described below.

First, in order to delete the NdeI recognition site (restriction site originated in the pUC18) contained in the plasmid pSF_Sm_Aet derived from pUC18 containing a peptide-producing enzyme gene derived from the Sphingobacterium sp. FERM BP-8124 and the phoC promoter (WO2006/075486 A1), PCR was performed by using pSF_Sm_Aet as the template and primers having the sequences of SEQ ID NOS: 24 and 25 with “Quik Change Site-Directed Mutagenesis Kit” of Stratagene according to the manufacturer's protocol. The obtained PCR product was digested with DpnI, and then the Escherichia coli JM109 strain was transformed with the reaction mixture, applied to the LB agar medium containing 100 mg/L of ampicillin sodium (Amp), and cultured at 25° C. for 36 hours. Plasmids were extracted from the grown colonies of the transformants in a known manner, the nucleotide sequences thereof were confirmed by using 3100 Genetic Analyzer (Applied Biosystems), and the plasmid having the objective structure was designated as pSF1_Sm_Aet. The FERM BP-8124 strain was designated as AJ110003, and deposited at the independent administrative agency, National Institute of Advanced Industrial Science and Technology, International Patent Organism Depository (Tsukuba Central 6, 1-1, Higashi 1-Chome, Tsukuba-shi, Ibaraki-ken, 305-8566, Japan) on Jul. 22, 2002 in line with the provisions of the Budapest Treaty, and assigned with an accession number of FERM BP-8124.

Then, in order to introduce the NdeI recognition sequence into pSF1_Sm_Aet at the site of the start methionine moiety of the peptide-producing enzyme gene derived from Sphingobacterium sp. FERM BP-8124, PCR was performed by using pSF1_Sm_Aet as the template and primers having the sequences of SEQ ID NOS: 26 and 27 with “Quik Change Site-Directed Mutagenesis Kit” mentioned above. The obtained PCR product was digested with DpnI, and then the Escherichia coli JM109 strain was transformed with the reaction mixture, applied to the LB agar medium containing 100 mg/L of Amp, and cultured at 25° C. for 24 hours. Plasmids were extracted from the grown colonies of the transformants in a known manner, the nucleotide sequences thereof were confirmed by using 3100 Genetic Analyzer (Applied Biosystems), and the plasmid having the objective structure was designated as pSF2_Sm_Aet.

Then, the phoC promoter of pSF2_Sm_Aet was replaced with the rpoH promoter according to the following method. The rpoH promoter region was obtained by PCR from the Escherichia coli W3110 strain genomic DNA. PCR was performed by using the W3110 strain genomic DNA as the template, a primer having the sequence of SEQ ID NO: 28 (rpoH promoter region having a nucleotide sequence containing the XbaI recognition sequence at the 5′ end) as the sense primer, a primer having the sequence of SEQ ID NO: 29 (complementary nucleotide sequence of the rpoH promoter region having a nucleotide sequence containing the NdeI recognition sequence at the 5′ end) as the antisense primer, and KOD-plus- (Toyobo) as the polymerase, with 30 cycles of 94° C. for 30 seconds, 52° C. for 1 minute, and 68° C. for 30 seconds according to the manufacturer's protocol.

Then, the obtained PCR product was digested with XbaI/NdeI, and subjected to agarose gel electrophoresis, a portion of the DNA of about 0.4 kb was excised, and the DNA was ligated to the pSF2_Sm_Aet fragment (about 4.7 kb) digested with XbaI/NdeI by using DNA Ligation Kit Ver. 2.1 (Takara Bio). The Escherichia coli JM109 strain was transformed with the reaction mixture, applied to the LB agar medium containing 100 mg/L of Amp, and cultured at 25° C. for 36 hours. Plasmids were extracted from the grown colonies of the transformants in a known manner, the nucleotide sequences thereof were confirmed by using 3100 Genetic Analyzer (Applied Biosystems), and the plasmid having the objective structure was designated as pSF12_Sm_Aet.

The ggt gene of the Escherichia coli was obtained by PCR from the Escherichia coli W3110 strain genomic DNA. PCR was performed by using the W3110 strain genomic DNA as the template, a primer having the sequence of SEQ ID NO: 30 (region containing the initiation codon of the ggt gene having a nucleotide sequence containing the NdeI recognition sequence at the 5′ end) as the sense primer, a primer having the sequence of SEQ ID NO: 31 (complementary nucleotide sequence of the region containing the initiation codon of the ggt gene having a nucleotide sequence containing the PstI recognition sequence at the 5′ end) as the antisense primer, and KOD-plus- (Toyobo), with 30 cycles of 94° C. for 30 seconds, 52° C. for 1 minute, and 68° C. for 120 seconds according to the manufacturer's protocol. Then, the obtained PCR product was digested with NdeI/PstI, and subjected to agarose gel electrophoresis, a portion of the objective DNA of about 1.8 kb was excised, and the DNA was ligated to the pSF12_Sm_Aet fragment digested with NdeI/PstI (about 3.0 kb) by using DNA Ligation Kit Ver. 2.1 (Takara Bio). The Escherichia coli JM109 strain was transformed with the reaction mixture, applied to the LB agar medium containing 100 mg/L of Amp, and cultured at 25° C. for 36 hours. Plasmids were extracted from the grown colonies of the transformants in a known manner, the nucleotide sequences thereof were confirmed by using 3100 Genetic Analyzer (Applied Biosystems), and the plasmid having the objective structure was designated as pSF12_ggt.

Example 2 Preparation of pepA Gene-, pepB Gene-, pepD Gene-, pepE Gene- and pepN Gene-Disrupted Strains Derived from Escherichia coli JM 109 Strain

From the Escherichia coli JM109 strain as a parent strain, PepA, PepB, PepD, PepE, and PepN non-producing strains were constructed. PepA is encoded by the pepA gene (GenBank Accession: 7439053, SEQ ID NO: 16), PepB is encoded by the pepB gene (GenBank Accession: 7437614, SEQ ID NO: 18), PepD is encoded by the pepD gene (GenBank Accession: 7438954, SEQ ID NO: 14), PepE is encoded by the pepE gene (GenBank Accession: 7438857, SEQ ID NO: 20), and PepN is encoded by the pepN gene (GenBank Accession: 7438913, SEQ ID NO: 22).

Each gene was disrupted by a method consisting of a combination of the method called “Red-driven integration”, first developed by Datsenko and Wanner (Proc. Natl. Acad. Sci. USA, vol. 97, No. 12, pp. 6640-6645, 2000), and an excision system derived from λ phage (J. Bacteriol., 2002 September, 184 (18):5200-3, Interactions between integrase and excisionase in the phage lambda excisive nucleoprotein complex, Cho E H, Gumport R I, and Gardner J F) (refer to WO2005/010175). According to the “Red-driven integration” method, using a PCR product obtained by using synthetic oligonucleotides in which a part of a target gene is designed on the 5′ side, and a part of antibiotic resistance gene is designed on the 3′ side, respectively, as primers, a gene-disrupted strain can be constructed in one step. By further using the excision system derived from λ phage in combination, the antibiotic resistance gene incorporated into the gene-disrupted strain can be eliminated.

As the template for PCR, the plasmid pMW118-attL-Cm-attR was used. pMW118-attL-Cm-attR (WO2006/078039) is a plasmid obtained by inserting attL and attR genes, which are the attachment sites of λ phage, and the cat gene, which is an antibiotic resistance gene, into pMW118 (Nippon Gene), and the genes are inserted in the order of attL-cat-attR. PCR was performed by using synthetic oligonucleotides as primers having sequences corresponding to the both ends of these attL and attR at the 3′ ends and a sequence corresponding to a part of the pepA, pepB, pepD, pepE or pepN gene as the objective gene at the 5′ ends.

That is, a DNA fragment for disruption of the pepA gene was prepared by performing PCR using pMW118-attL-Cm-attR as the template, the primers having the sequences of SEQ ID NOS: 32 and 33, and KOD-plus- of Toyobo, with 30 cycles of 94° C. for 30 seconds, 52° C. for 1 minute, and 68° C. for 120 seconds according to the manufacturer's protocol.

The DNA fragment for disruption of the pepB gene was obtained as follows. Namely, a fragment of about 1.0 kb locating upstream of the pepB gene was amplified by performing PCR using the Escherichia coli W3110 strain genomic DNA as the template, the primers having the sequences of SEQ ID NOS: 34 and 35, and KOD-plus- with 30 cycles of 94° C. for 30 seconds, 52° C. for 1 minute, and 68° C. for 60 seconds according to the manufacturer's protocol (DNA fragment A). Similarly, a fragment of about 1.0 kb locating downstream of the pepB gene was amplified by performing PCR using the primers having the sequences of SEQ ID NOS: 36 and 37, and KOD-plus- with 30 cycles of 94° C. for 30 seconds, 52° C. for 1 minute, and 68° C. for 60 seconds according to the manufacturer's protocol (DNA fragment B). Further, a fragment of about 1.6 kb was amplified by performing PCR using the plasmid pMW118-attL-Cm-attR as the template, the primers having the sequences of SEQ ID NOS: 38 and 39, and KOD-plus- with 30 cycles of 94° C. for 30 seconds, 52° C. for 1 minute, and 68° C. for 120 seconds according to the manufacturer's protocol (DNA fragment C). By using the obtained DNA fragments A, B and C in amounts of 50, 10, and 50 ng, respectively, PCR was performed by using KOD-plus- with 10 cycles of 94° C. for 2 minutes, 52° C. for 30 seconds, and 68° C. for 2 minutes according to the manufacturer's protocol. Then, second PCR was performed by using 1 μl of each of the obtained PCR products as the template together with the primers having the sequences of SEQ ID NOS: 34 and 37, and KOD-plus- with 30 cycles of 94° C. for 30 seconds, 52° C. for 1 minute, and 68° C. for 4 minutes according to the manufacturer's protocol to obtain the DNA fragment for disruption of the pepB gene.

The DNA fragment for disruption of the pepD gene was prepared by performing PCR using the primers having the sequences of SEQ ID NOS: 40 and 41 and KOD-plus- with 30 cycles of 94° C. for 30 seconds, 52° C. for 1 minute, and 68° C. for 120 seconds according to the manufacturer's protocol.

The DNA fragment for disruption of the pepE gene was prepared by performing PCR using the primers having the sequences of SEQ ID NOS: 42 and 43 and KOD-plus- with 30 cycles of 94° C. for 30 seconds, 52° C. for 1 minute, and 68° C. for 120 seconds according to the manufacturer's protocol.

The DNA fragment for disruption of the pepN gene was prepared by performing PCR using the primers having the sequences of SEQ ID NOS: 44 and 45 and KOD-plus- with 30 cycles of 94° C. for 30 seconds, 52° C. for 1 minute, and 68° C. for 120 seconds according to the manufacturer's protocol.

The DNA fragments for gene disruption obtained as described above were each purified by agarose gel electrophoresis, and introduced into the Escherichia coli JM109 strain harboring the plasmid pKD46 having temperature sensitive replication ability by electroporation. The plasmid pKD46 (Proc. Natl. Acad. Sci. USA, 97:12:6640-45, 2000) includes a total 2,154 nucleotide DNA fragment of phage λ (GenBank/EMBL accession no. J02459, nucleotide positions 31088 to 33241) containing genes coding for the Red recombinase of the λ Red homologous recombination system (γ, β, exo genes) under the control of the arabinose-inducible P_(araB) promoter. The plasmid pKD46 is necessary for integration of the DNA fragments for gene disruption into the chromosome of the JM109 strain. Competent cells for electroporation were prepared as follows. Namely, Escherichia coli JM109 strain harboring the plasmid pKD46 was cultured at 30° C. for 20 hours in the LB medium containing 100 mg/L of Amp, and the culture was diluted 50 times with 2 ml of the SOB medium (Molecular Cloning A Laboratory Manual, 2nd edition, Sambrook, J. et al., Cold Spring Harbor Laboratory Press (1989)) containing Amp (100 mg/L). The cells in the obtained diluted suspention were grown at 30° C. to an OD₆₀₀ of about 0.3, then added with 70 μl of 10% (v/v) L-arabinose, and cultured at 37° C. for 1 hour. Then, the obtained culture fluid was concentrated 65 times, and the cells were washed three times with 10% (v/v) glycerol and thereby made electrocompetent. Electroporation was performed by using 30 μl of the competent cells and about 100 ng of the PCR product.

After the electroporation, 0.27 mL of the SOC medium (Molecular Cloning A Laboratory Manual, 2nd edition, Sambrook, J. et al., Cold Spring Harbor Laboratory Press (1989)) was added to the cell suspension, and the cells were cultured at 37° C. for 3 hours, and then cultured at 37° C. on the LB agar medium containing chloramphenicol (Cm, 50 mg/L), and Cm resistant recombinant strains were chosen. Then, in order to remove the pKD46 plasmid, the strains were cultured at 42° C. on the LB agar medium containing Cm (50 mg/L), the obtained colonies were examined for the Amp resistance, and the Amp sensitive strains where pKD46 had been removed were obtained. Disruption of the pepA gene, pepB gene, pepD gene, pepE gene, and pepN gene of the mutants identified with the Cm resistance gene was confirmed by PCR. The obtained pepA gene-, pepB gene-, pepD gene-, pepE gene-, and pepN gene-disrupted strains were designated as JM109ΔpepA:att-cat strain, JM109ΔpepB:att-cat strain, JM109ΔpepD:att-cat strain, JM109ΔpepE:att-cat strain, and JM109ΔpepN:att-cat strain, respectively.

Then, in order to remove the att-cat gene introduced into the pepA gene, pepB gene, pepD gene, pepE gene, and pepN gene, pMW-intxis-ts was used as the helper plasmid. pMW-intxis-ts is a plasmid carrying a gene coding for phage integrase (Int) and a gene coding for excisionase (Xis), and having temperature sensitive replication ability. If pMW-intxis-ts is introduced into a cell, it recognizes attL or attR on the chromosome to cause recombination, thus the gene between attL and attR is excised, and only the attB sequence remains on the chromosome. Competent cells of the JM109ΔpepA:att-cat strain, JM109ΔpepB:att-cat strain, JM109ΔpepD:att-cat strain, JM109ΔpepE:att-cat strain, and JM109ΔpepN:att-cat strain obtained above were prepared in a conventional manner, transformed with pMW-intxis-ts, and cultured at 30° C. on the LB agar medium containing 100 mg/L of Amp, and Amp resistant strains were chosen. Then, in order to remove the pMW-intxis-ts plasmid, the transformants were cultured at 42° C. on the LB agar medium, and Amp resistance and Cm resistance of the obtained colonies were examined to obtain Cm and Amp sensitive strains, which were strains where att-cat and pMW-intxis-ts had been removed, and the pepA gene, the pepB gene, the pepD gene, the pepE gene, or the pepN gene had been disrupted. These strains were designated as JM109ΔpepA strain, JM109ΔpepB strain, JM109ΔpepD strain, JM109ΔpepE strain, and JM109ΔpepN strain, respectively.

Example 3 Construction of Mutant ggt Genes

In order to construct mutant ggt genes, PCR was performed by using primers corresponding to various mutant ggt genes (SEQ ID NOS: 46 to 211) and pSF12_ggt mentioned in Example 1 as the template with “Quik change Site-Directed Mutagenesis Kit” of Stratagene according to the manufacturer's protocol. The relations between the mutations and the primers are shown in Tables 2 to 5.

After each of the obtained PCR products was digested with DpnI, the Escherichia coli JM109ΔpepA strain, JM109ΔpepB strain, JM109ΔpepD strain, JM109ΔpepE strain, and JM109ΔpepN strain were each transformed with the reaction mixture, applied to the LB agar medium containing 100 mg/L of Amp, and cultured at 25° C. for 36 hours. Plasmids were extracted from the grown colonies of the transformants in a known manner, the nucleotide sequences thereof were confirmed by using 3100 Genetic Analyzer (Applied Biosystems), and the objective transformants were used for further examination.

The plasmids introduced with various mutations were given with designations consisting of pSF12-ggt and indication of type of the mutation. For example, a plasmid having a mutant ggt gene coding for a mutant GGT having the Y444E mutation is described as pSF12-ggt(Y444E).

TABLE 2 SEQ  Introduced ID NO Sequence mutation 46 CCGAACGTTGCGGGGCTGGTGG Y444A 47 CCACCAGCCCCGCAACGTTCGG 48 CCGAACGTTGATGGGCTGGTGG Y444D 49 CCACCAGCCCATCAACGTTCGG 50 CCGAACGTTGAAGGGCTGGTGG Y444E 51 CCACCAGCCCTTCAACGTTCGG 52 CCGAACGTTAACGGGCTGGTGG Y444N 53 CCACCAGCCCGTTAACGTTCGG 54 GCTTAATAACGCGATGGATGATTTC Q430A 55 GAAATCATCCATCGCGTTATTAAGC 56 GCTTAATAACTGCATGGATGATTTC Q430C 57 GAAATCATCCATGCAGTTATTAAGC 58 GCTTAATAACGATATGGATGATTTC Q430D 59 GAAATCATCCATATCGTTATTAAGC 60 GCTTAATAACGAAATGGATGATTTC Q430E 61 GAAATCATCCATTTCGTTATTAAGC 62 GCTTAATAACTTTATGGATGATTTC Q430F 63 GAAATCATCCATAAAGTTATTAAGC 64 GCTTAATAACGGCATGGATGATTTC Q430G 65 GAAATCATCCATGCCGTTATTAAGC 66 GCTTAATAACCATATGGATGATTTC Q430H 67 GAAATCATCCATATGGTTATTAAGC 68 GCTTAATAACATTATGGATGATTTC Q430I 69 GAAATCATCCATAATGTTATTAAGC 70 GCTTAATAACAAAATGGATGATTTC Q430K 71 GAAATCATCCATTTTGTTATTAAGC 72 GCTTAATAACCTGATGGATGATTTC Q430L 73 GAAATCATCCATCAGGTTATTAAGC 74 GCTTAATAACATGATGGATGATTTC Q430M 75 GAAATCATCCATCATGTTATTAAGC 76 GCTTAATAACAACATGGATGATTTC Q430N 77 GAAATCATCCATGTTGTTATTAAGC 78 GCTTAATAACCCGATGGATGATTTC Q430P 79 GAAATCATCCATCGGGTTATTAAGC 80 GCTTAATAACCGCATGGATGATTTC Q430R 81 GAAATCATCCATGCGGTTATTAAGC 82 GCTTAATAACAGCATGGATGATTTC Q430S 83 GAAATCATCCATGCTGTTATTAAGC 84 GCTTAATAACACCATGGATGATTTC Q430T 85 GAAATCATCCATGGTGTTATTAAGC 86 GCTTAATAACGTGATGGATGATTTC Q430V 87 GAAATCATCCATCACGTTATTAAGC 88 GCTTAATAACTGGATGGATGATTTC Q430W 89 GAAATCATCCATCCAGTTATTAAGC 90 GCTTAATAACTATATGGATGATTTC Q430Y 91 GAAATCATCCATATAGTTATTAAGC 92 CCAGATGGATGCGTTCTCCGCC D433A 93 GGCGGAGAACGCATCCATCTGG

TABLE 3 SEQ  Introduced ID NO Sequence mutation 94 CCAGATGGATTGCTTCTCCGCC D433C 95 GGCGGAGAAGCAATCCATCTGG 96 CCAGATGGATGAATTCTCCGCC D433E 97 GGCGGAGAATTCATCCATCTGG 98 CCAGATGGATTTTTTCTCCGCC D433F 99 GGCGGAGAAAAAATCCATCTGG 100 CCAGATGGATGGCTTCTCCGCC D433G 101 GGCGGAGAAGCCATCCATCTGG 102 CCAGATGGATCATTTCTCCGCC D433H 103 GGCGGAGAAATGATCCATCTGG 104 CCAGATGGATATTTTCTCCGCC D433I 105 GGCGGAGAAAATATCCATCTGG 106 CCAGATGGATAAATTCTCCGCC D433K 107 GGCGGAGAATTTATCCATCTGG 108 CCAGATGGATCTGTTCTCCGCC D433L 109 GGCGGAGAACAGATCCATCTGG 110 CCAGATGGATATGTTCTCCGCC D433M 111 GGCGGAGAACATATCCATCTGG 112 CCAGATGGATAACTTCTCCGCC D433N 113 GGCGGAGAAGTTATCCATCTGG 114 CCAGATGGATCCGTTCTCCGCC D433P 115 GGCGGAGAACGGATCCATCTGG 116 CCAGATGGATCAGTTCTCCGCC D433Q 117 GGCGGAGAACTGATCCATCTGG 118 CCAGATGGATCGCTTCTCCGCC D433R 119 GGCGGAGAAGCGATCCATCTGG 120 CCAGATGGATAGCTTCTCCGCC D433S 121 GGCGGAGAAGCTATCCATCTGG 122 CCAGATGGATACCTTCTCCGCC D433T 123 GGCGGAGAAGGTATCCATCTGG 124 CCAGATGGATGTGTTCTCCGCC D433V 125 GGCGGAGAACACATCCATCTGG 126 CCAGATGGATTGGTTCTCCGCC D433W 127 GGCGGAGAACCAATCCATCTGG 128 CCAGATGGATTATTTCTCCGCC D433Y 129 GGCGGAGAAATAATCCATCTGG 130 GCAGCCCGCGGCGAAACTGGCACG F174A 131 CGTGCCAGTTTCGCCGCGGGCTGC 132 GCAGCCCGCGATTAAACTGGCACG F174I 133 CGTGCCAGTTTAATCGCGGGCTGC 134 GCAGCCCGCGCTGAAACTGGCACG F174L 135 CGTGCCAGTTTCAGCGCGGGCTGC 136 GCAGCCCGCGATGAAACTGGCACG F174M 137 CGTGCCAGTTTCATCGCGGGCTGC

TABLE 4 SEQ Introduced ID NO Sequence mutation 138 GCAGCCCGCGGTGAAACTGGCACG F174V 139 CGTGCCAGTTTCACCGCGGGCTGC 140 GCAGCCCGCGIGGAAACTGGCACG F174W 141 CGTGCCAGTTTCCACGCGGGCTGC  142 GCAGCCCGCGTATAAACTGGCACG F174Y 143 CGTGCCAGTTTATACGCGGGCTGC 144 GTGGTGAAAATTGGTAAAACCTG D472I 145 CAGGTTTTACCAATTTTCACCAC 146 CTATAAAGGCCGCATTGCGGAAC T246R 147 GTTCCGCAATGCGGCCTTTATAG 148 GATCCATATCCTGCAAATCCTCAATATTC V301L 149 GAATATTGAGGATTTGCAGGATATGGATC 150 GCTGAACACCAACTTCGGTACGG T413N 151 CCGTACCGAAGTTGGTGTTCAGC 152 GTAGCCCAGGCAGCAGCCGGATCATC G484S 153 GATGATCCGGCTGCTGCCTGGGCTAC 154 GTGATGCCAACAGCGTCGGGCCGAAC A453S 155 GTTCGGCCCGACGCTGTTGGCATCAC 156 CGTACCGAACGCGGAAGGGCTGG V443A/Y444E 157 CCAGCCCTTCCGCGTTCGGTACG 158 GCTGAACACCGCGTTCGGTACGG T413A 159 CCGTACCGAACGCGGTGTTCAGC 160 GCTGAACACCCATTTCGGTACGG T413H 161 CCGTACCGAAATGGGTGTTCAGC 162 GTAGCCCAGGCGCGAGCCGGATCATC G484A 163 GATGATCCGGCTCGCGCCTGGGCTAC 164 GTAGCCCAGGCGAAAGCCGGATCATC G484E 165 GATGATCCGGCTTTCGCCTGGGCTAC 166 GGTGTGGAGAAAGATGTCTTC E38K 167 GAAGACATCTTTCTCCACACC 168 CGATATGTTCGTGGATGATCAGG L127V 169 CCTGATCATCCACGAACATATCG 170 GGTGGTGAATTGCATCGATTATG S498C 171 CATAATCGATGCAATTCACCACC 172 GAAGCAAAAGGTCATAAAGTGGCGC Q542H 173 GCGCCACTTTATGACCTTTTGCTTC 174 GGTCGAACGCAACCCGATAAGCG T276N 175 CGCTTATCGGGTTGCGTTCGACC 176 CCGACCCGCGCAAAGTGGATGATTTAAC S572K 177 GTTAAATCATCCACTTTGCGCGGGTCGG

TABLE 5 SEQ Introduced ID NO Sequence mutation 178 CTATACGCTGCAGACCACCTTCG N411Q 179 CGAAGGTGGTCTGCAGCGTATAG 180 CCGGGCGTAGCGAACGTTGAAG P441A/Y444E 181 CTTCAACGTTCGCTACGCCCGG  182 GTACCGAACGAAGAAGGGCTGG V443E/Y444E 183 CCAGCCCTTCTTCGTTCGGTAC 184 GTACCGAACGGCGAAGGGCTGG V443G/Y444E 185 CCAGCCCTTCGCCGTTCGGTAC 186 GTACCGAACCTGGAAGGGCTGG V443L/Y444E 187 CCAGCCCTTCCAGGTTCGGTAC  188 GTACCGAACAACGAAGGGCTGG  V443N/Y444E 189 CAGCCCTTCGTTGTTCGGTAC 190 GTACCGAACCAGGAAGGGCTGG V443Q/Y444E 191 CCAGCCCTTCCTGGTTCGGTAC 192 GTTGAAGGGGCGGTGGGCGGTG Y444E/L446A 193 CACCGCCCACCGCCCCTTCAAC 194 GGTTGGGCCGAACGGTGAGTTGTAC D561N 195 GTACAACTCACCGTTCGGCCCAACC 196 GCCGCCGCGCATCCTGCGCCGCC P27H 197 GGCGGCGCAGGATGCGCGGCGGC 198 GTAATCAAACTTGCCATTACTCAGTG T392C 199 CACTGAGTAATGGCAAGTTTGATTAC

Example 4 Escherichia coli Strains Deficient in Each Peptidase and Evaluation of Val-Gly Degradation Ability Thereof

The JM109ΔpepA strain, JM109ΔpepB strain, JM109ΔpepD strain, JM109ΔpepE strain, and JM109ΔpepN strain were transformed with pUC18, respectively.

Each of the obtained transformants was cultured at 25° C. for 22 hours using the LB medium [1.0% (w/v) peptone, 0.5% (w/v) yeast extract, and 1.0% (w/v) NaCl] containing 100 mg/L of Amp. The cells in the obtained culture fluid were washed with a 0.2 M potassium phosphate buffer (pH 8.0), and a cell suspension was prepared with the same buffer. A reaction mixture containing 100 mM Val-Gly, 0.2 M potassium phosphate buffer (pH 8.0), and the cells was prepared. The cell density was such a density that the reaction mixture diluted 51 times showed an absorbance of 0.2 at 610 nm. When a metal salt was added to the reaction mixture, it was added at a final concentration of 0.1 mM. When ethylenediaminetetraacetic acid (EDTA) was added, it was added by using a 500 mM aqueous solution thereof produced by Nakarai Tesque (pH 8.0) at a final concentration of 1 mM. The reaction conditions were 20° C. and 20 hours, and Val-Gly was quantified by HPLC after completion of the reaction. The quantification conditions were as follows.

As the column, Synergi 4μ Hydro-RP 80A produced by Phenomenex (particle size: 4 microns, internal diameter: 4.6 mm, length: 250 mm) was used. As the eluent, Solution A (50 mM sodium dihydrogenphosphate, pH 2.5, pH was adjusted with phosphoric acid) and Solution B (1:1 mixture of Solution A and acetonitrile) were used. The column temperature was 40° C., and the detection UV wavelength was 210 nm. As the gradient of the eluent, used were 0 to 5% Solution B for 0 to 5 minutes, 5% Solution B for 5 to 15 minutes, 5 to 80% Solution B for 15 to 30 minutes, 80 to 0% Solution B for 30 to 30.1 minutes, and 0% Solution B for 30.1 to 50 minutes.

The results are shown in Table 6.

TABLE 6 Host bacterium strain JM109 JM109 JM109 JM109 JM109 JM109 Deficient gene — ΔpepA ΔpepB ΔpepD ΔpepE ΔpepN Harbored plasmid pUC18 pUC18 pUC18 pUC18 pUC18 pUC18 No addition 89.4 85.8 84.5 93.8 81.4 61.0 0.1 mM CoCl₂ 0.0 0.0 0.0 86.8 0.0 0.0 0.1 mM MgCl₂ 84.7 78.5 79.7 85.6 78.8 58.5 0.1 mM 0.0 0.0 0.0 85.7 0.0 0.0 MnSO₄ 0.1 mM NiSO₄ 80.5 71.6 75.5 87.3 75.1 0.0 0.1 mM ZnSO₄ 74.7 73.4 71.7 93.2 73.8 0.0 0.1 mM FeSO₄ 0.0 0.0 0.0 90.3 0.0 0.0   1 mM EDTA 94.0 91.9 91.2 91.3 90.1 90.1

As shown in Table 2, the cells of Escherichia coli wild-type strain decomposed Val-Gly in the presence of Co²⁺, Mn²⁺, or Fe²⁺ ions. These results revealed that PepD mainly participates in the decomposition of Val-Gly. In addition, when these metal ions were not added, decomposition of Val-Gly was suppressed to some extent by addition of 1 mM EDTA.

Example 5 Evaluation of γ-glutamylation of Val-Gly by Mutant GGT Enzymes

The JM109ΔpepD strain was transformed with the plasmids described in Example 3. The transformants were cultured at 25° C. for 22 hours by using the TB medium [Terrific Broth, Molecular Cloning A Laboratory Manual, 3rd edition, Sambrook, J. et al., Cold Spring Harbor Laboratory Press (2001)] containing 100 mg/L of Amp. Each of the obtained culture fluids and an equivalent volume of a test solution (0.2 M L-glutamine, 0.2 M Val-Gly, pH of the test solution was adjusted to pH 7.6 with NaOH) were mixed to start the reaction. The reaction conditions were 20° C. and 4 hours, and γ-Glu-Val-Gly was quantified by HPLC after completion of the reaction. HPLC was performed under the same conditions as those of Example 4.

The results are shown in Tables 7 to 9.

TABLE 7 Amino acid substitution γ-Glu-Val-Gly 1 2 3 4 (mM) 4.8 E38 K 5.8 F174 I 4.8 N411 Q 43.4 T392 C 20.0 T413 N 4.2 T413 H 7.0 Q430 A 5.3 Q430 C 9.6 Q430 D 11.7 Q430 E 9.9 Q430 F 8.2 Q430 G 4.7 Q430 H 9.6 Q430 I 9.0 Q430 K 9.2 Q430 L 9.1 Q430 M 40.8 Q430 N 6.8 Q430 P 8.3 Q430 R 1.3 Q430 S 9.5 Q430 T 4.6 Q430 V 5.4 Q430 W 3.8 Q430 Y 14.6 D433 A 10.8 D433 C 0.3 D433 E 38.6 D433 F 0.1 D433 G 19.0 D433 H 0.2 D433 I 0.1 D433 K 0.1 D433 L 0.1 D433 M 0.3 D433 N 5.7 D433 P 0.2 D433 Q 0.2 D433 R 0.1 D433 S 5.6 D433 T 0.2 D433 V 0.1 D433 W 0.1

TABLE 8 Amino acid substitution γ-Glu-Val-Gly 1 2 3 4 (mM) D433 Y 0.2 Y444 A 52.5 Y444 D 62.4 Y444 E 63.3 S572 K 25.9 G484 E 6.9 G484 S 47.9 E38 K Y444 E 65.3 F174 A Y444 E 61.9 F174 I Y444 E 62.6 F174 L Y444 E 64.8 F174 M Y444 E 65.2 F174 V Y444 E 66.6 F174 W Y444 E 62.8 F174 Y Y444 E 65.2 T246 R Y444 E 66.0 V301 L Y444 E 66.1 T392 C Y444 A 54.0 T392 C Y444 E 19.9 N411 Q Q430 N 21.3 N411 Q D433 E 0.0 N411 Q S572 K 40.9 T413 N Q430 N 4.1 T413 N G484 E 17.1 T413 N G484 S 22.4 T413 N S572 K 4.4 T413 A Y444 E 64.0 T413 H Y444 E 70.8 T413 N Y444 E 65.4 Q430 N Y444 A 53.6 Q430 N Y444 E 63.2 Q430 N Y444 D 64.7 Q430 N Y444 N 61.9 D433 E Y444 A 9.3 D433 E Y444 E 8.5 P441 A Y444 E 63.1 V443 A Y444 E 68.4 V443 E Y444 E 60.8 V443 G Y444 E 67.4 V443 L Y444 E 61.3 V443 N Y444 E 60.8 V443 Q Y444 E 64.4 Y444 E L446 A 64.5 Y444 E A453 S 64.2 Y444 E D472 I 64.1

TABLE 9 Amino acid substitution γ-Glu-Val-Gly 1 2 3 4 (mM) Y444 E G484 A 65.5 Y444 E G484 S 60.1 Y444 E S498 C 66.7 Y444 E Q542 H 66.1 Y444 E D561 N 65.4 P27 H T413 N Y444 E 64.9 P27 H Y444 E G484 S 62.1 E38 K T413 N Y444 E 62.6 L127 V T413 N Y444 E 62.8 T276 N T413 N Y444 E 63.1 T276 N Y444 E G484 S 64.5 T413 H Y444 E G484 S 66.5 T413 N Q430 N Y444 E 65.8 T413 N V443 A Y444 E 62.8 T413 N Y444 E A453 S 62.6 T413 N Y444 E S498 C 62.6 T413 N Y444 E Q542 H 64.0 T413 N Y444 E G484 S 60.3 V443 A Y444 E G484 S 65.0 Y444 E G484 S Q542 H 63.4 T413 N V443 A Y444 E G484 S 62.4 T413 N Y444 E A453 S G484 S 61.2 T413 N Y444 E G484 S Q542 H 61.3 T413 N Y444 E G484 S S572 K 62.1 T413 N Q430 N Y444 E G484 S 69.9 T413 N Y444 E G484 E S498 C 60.8

Example 6 Evaluation of γ-Glutamylation of Val-Gly by Mutant GGT Enzyme-Expressing Strains Prepared by Using Escherichia coli Strains Deficient in Each Peptidase as Host

The Escherichia coli JM109 strain, JM109ΔpepA strain, JM109ΔpepB strain, JM109ΔpepD strain, JM109ΔpepE strain, and JM109ΔpepN strain were transformed with pUC18, pSF12-ggt, or pSF12-ggt (Y444E) to obtain transformants. Each of the obtained transformants was cultured at 25° C. for 20 hours using the LB medium [1.0% (w/v) peptone, 0.5% (w/v) yeast extract, and 1.0% (w/v) NaCl] containing 100 mg/L of Amp. The obtained culture fluid was centrifuged to separate the culture fluid into wet cells and supernatant, and the wet cells were suspended in the supernatant to prepare a cell suspension so that the suspension diluted 51 times had an absorbance of 0.4 at 610 nm. This cell suspension and an equivalent volume of a test solution (0.2 M potassium phosphate buffer (pH 8.0), 0.2 M L-glutamine, 0.2 M Val-Gly) were mixed to start the reaction. The reaction conditions were 20° C. and 20 hours, and γ-Glu-Val-Gly and Val-Gly were quantified by HPLC after completion of the reaction under the same conditions as those of Example 4. The results are shown in Table 10.

TABLE 10 Val-Gly γ-Glu-Val-Gly Strain Harbored plasmid (mM) (mM) JM109 pSF12-ggt 59.2 1.8 JM109 ΔpepA pSF12-ggt 62.8 1.8 JM109 ΔpepB pSF12-ggt 59.6 1.9 JM109 ΔpepD pSF12-ggt 85.1 2.9 JM109 ΔpepE pSF12-ggt 59.3 1.7 JM109 ΔpepN pSF12-ggt 60.7 2.1 JM109 pSF12-ggt (Y444E) 38.4 34.5 JM109 ΔpepA pSF12-ggt (Y444E) 42.7 33.3 JM109 ΔpepB pSF12-ggt (Y444E) 39.4 31.0 JM109 ΔpepD pSF12-ggt (Y444E) 48.3 36.1 JM109 ΔpepE pSF12-ggt (Y444E) 36.7 31.2 JM109 ΔpepN pSF12-ggt (Y444E) 38.3 30.6 Explanation of Sequence Listing SEQ ID NO: 1: Nucleotide sequence of Escherichia coli ggt gene SEQ ID NO: 2: Amino acid sequence of Escherichia coli GGT SEQ ID NO: 3: Amino acid sequence of Shigella flexneri flexneri 5 str. 8401 GGT SEQ ID NO: 4: Amino acid sequence of Shigella dysenteriae Sd197 GGT SEQ ID NO: 5: Amino acid sequence of Shigella boydii strain Sb227 GGT SEQ ID NO: 6: Amino acid sequence of Salmonella Typhimurium ATCC 700720 GGT SEQ ID NO: 7: Amino acid sequence of Salmonella enterica enterica choleraesuis strain SC-B67 GGT SEQ ID NO: 8: Amino acid sequence of Salmonella enterica typhi strain Ty2 GGT SEQ ID NO: 9: Amino acid sequence of Klebsiella pneumoniae ATCC 202080 GGT SEQ ID NO: 10: Amino acid sequence of Salmonella enterica subsp. enterica serovar A str. ATCC 9150 Paratyphi GGT SEQ ID NO: 11: Amino acid sequence of Klebsiella pneumoniae clone KPN308894 GGT SEQ ID NO: 12: Amino acid sequence of Enterobacter cloaceae clone EBC103795 GGT SEQ ID NO: 13: Consensus sequence of various GGT small subunits SEQ ID NO: 14: Nucleotide sequence of Escherichia coli pepD gene SEQ ID NO: 15: Amino acid sequence of Escherichia coli PepD SEQ ID NO: 16: Nucleotide sequence of Escherichia coli pepA gene SEQ ID NO: 17: Amino acid sequence of Escherichia coli PepA SEQ ID NO: 18: Nucleotide sequence of Escherichia coli pepB gene SEQ ID NO: 19: Amino acid sequence of Escherichia coli PepB SEQ ID NO: 20: Nucleotide sequence of Escherichia coli pepE gene SEQ ID NO: 21: Amino acid sequence of Escherichia coli PepE SEQ ID NO: 22: Nucleotide sequence of Escherichia coli pepN gene SEQ ID NO: 23: Amino acid sequence of Escherichia coli PepN SEQ ID NOS: 24 to 31: PCR primers for preparation of pSF12_ggt SEQ ID NOS: 32 to 45: PCR primers for disruption of various peptidase genes SEQ ID NOS: 46 to 199: PCR primers for introduction of mutation

INDUSTRIAL APPLICABILITY

The mutant GGT of the present invention has a high activity for catalyzing γ-glutamylation of Val-Gly. Therefore, according to the method for producing γ-Glu-Val-Gly using the mutant GGT of the present invention, γ-Glu-Val-Gly can be efficiently produced by using Val-Gly as a raw material. 

What is claimed is:
 1. A method for producing γ-Glu-Val-Gly comprising reacting Val-Gly with a γ-glutamyl group donor in the presence of a γ-glutamyltransferase such that γ-Glu-Val-Gly is generated, wherein the γ-glutamyltransferase comprises a large subunit and a small subunit, wherein the large subunit has an amino acid sequence having no less than 90% identity to the amino acid sequence of the positions 26 to 390 of the amino acid sequence of SEQ ID NO: 2, wherein the glutamic acid at the position corresponding to position 38 of SEQ ID NO: 2 is modified by a substitution, wherein the small subunit has an amino acid sequence having no less than 90% sequence identity to the amino acid sequence of the positions 391 to 580 of the amino acid sequence of SEQ ID NO: 2, and wherein the tyrosine at the position corresponding to position 444 of SEQ ID NO: 2 is modified by a substitution.
 2. The method according to claim 1, wherein the tyrosine at the position corresponding to position 444 of SEQ ID NO: 2 is substituted with aspartic acid, glutamic acid, asparagine, or alanine.
 3. The method according to claim 1, wherein the small subunit has an amino acid substitution corresponding to a substitution in SEQ ID NO: 2 selected from the group consisting of Y444D, Y444E, Y444A, (T413A+Y444E), (T413H+Y444E), (T413N+Y444E), (Q430N+Y444E), (Q430N+Y444D), (Q430N+Y444N), (P441A+Y444E), (V443A+Y444E), (V443E+Y444E), (V443G+Y444E), (V443L+Y444E), (V443N+Y444E), (V443Q+Y444E), (Y444E+L446A), (Y444E+A453S), (Y444E+D472I), (Y444E+G484A), (Y444E+G484S), (Y444E+S498C), (Y444E+Q542H), (Y444E+D561N), (T413N+Y444E+V443A), (T413N+Y444E+A453S), (T413N+Y444E+S498C), (T413N+Y444E+Q542H), (G484S+Y444E+V443A), (G484S+Y444E+Q542H), (Q430N+Y444E+T413N), (T413H+Y444E+G484S), (T413N+Y444E+G484S), (T413N+Y444E+G484S+V443A), (T413N+Y444E+G484S+A453S), (T413N+Y444E+G484S+Q542H), (T413N+Y444E+G484S+S572K), (T413N+Y444E+G484S+Q430N), and (T413N+Y444E+G484E+S498C).
 4. The method according to claim 1, wherein the glutamic acid at the position corresponding to position 38 of SEQ ID NO: 2 is substituted with lysine.
 5. The method according to claim 1, wherein the small subunit has the amino acid sequence of SEQ ID NO: 13 except that the tyrosine at the position 54 of SEQ ID NO: 13 corresponding to position 444 of SEQ ID NO: 2 is modified by a substitution.
 6. The method according to claim 1, wherein the small subunit has the amino acid sequence of the positions 391 to 580 of SEQ ID NO: 2 except that the tyrosine at the position 444 of SEQ ID NO: 2 is substituted with aspartic acid, glutamic acid, asparagine, or alanine, the positions 391 to 580 of SEQ ID NO: 3 except that the tyrosine at the position 444 of SEQ ID NO: 3 corresponding to the position 444 of SEQ ID NO: 2 is substituted with aspartic acid, glutamic acid, asparagine, or alanine, the positions 392 to 581 of SEQ ID NO: 4 except that the tyrosine at the position 445 of SEQ ID NO: 4 corresponding to the position 444 of SEQ ID NO: 2 is substituted with aspartic acid, glutamic acid, asparagine, or alanine, the positions 388 to 577 of SEQ ID NO: 5 except that the tyrosine at the position 441 of SEQ ID NO: 5 corresponding to the position 444 of SEQ ID NO: 2 is substituted with aspartic acid, glutamic acid, asparagine, or alanine, the positions 391 to 580 of SEQ ID NO: 6 except that the tyrosine at the position 444 of SEQ ID NO: 6 corresponding to the position 444 of SEQ ID NO: 2 is substituted with aspartic acid, glutamic acid, asparagine, or alanine, the positions 391 to 580 of SEQ ID NO: 7 except that the tyrosine at the position 444 of SEQ ID NO: 7 corresponding to the position 444 of SEQ ID NO: 2 is substituted with aspartic acid, glutamic acid, asparagine, or alanine, the positions 391 to 580 of SEQ ID NO: 8 except that the tyrosine at the position 444 of SEQ ID NO: 8 corresponding to the position 444 of SEQ ID NO: 2 is substituted with aspartic acid, glutamic acid, asparagine, or alanine, the positions 400 to 589 of SEQ ID NO: 9 except that the tyrosine at the position 453 of SEQ ID NO: 9 corresponding to the position 444 of SEQ ID NO: 2 is substituted with aspartic acid, glutamic acid, asparagine, or alanine, the positions 391 to 580 of SEQ ID NO: 10 except that the tyrosine at the position 444 of SEQ ID NO: 10 corresponding to the position 444 of SEQ ID NO: 2 is substituted with aspartic acid, glutamic acid, asparagine, or alanine, the positions 392 to 581 of SEQ ID NO: 11 except that the tyrosine at the position 445 of SEQ ID NO: 11 corresponding to the position 444 of SEQ ID NO: 2 is substituted with aspartic acid, glutamic acid, asparagine, or alanine, or the positions 392 to 581 of SEQ ID NO: 12 except that the tyrosine at the position 445 of SEQ ID NO: 12 corresponding to the position 444 of SEQ ID NO: 2 is substituted with aspartic acid, glutamic acid, asparagine, or alanine.
 7. The method according to claim 1, wherein the large subunit has the amino acid sequence of: the positions 26 to 390 of SEQ ID NO: 2 except that the glutamic acid at the position 38 of SEQ ID NO: 2 is substituted with lysine, the positions 26 to 390 of SEQ ID NO: 3 except that the glutamic acid at the position 38 of SEQ ID NO: 3 corresponding to the position 38 of SEQ ID NO: 2 is substituted with lysine, the positions 26 to 391 of SEQ ID NO: 4 except that the glutamic acid at the position 39 of SEQ ID NO: 4 corresponding to the position 38 of SEQ ID NO: 2 is substituted with lysine, the positions 26 to 387 of SEQ ID NO: 5 except that the glutamic acid at the position 35 of SEQ ID NO: 5 corresponding to the position 38 of SEQ ID NO: 2 is substituted with lysine, the positions 25 to 390 of SEQ ID NO: 6 except that the glutamic acid at the position 38 of SEQ ID NO: 6 corresponding to the position 38 of SEQ ID NO: 2 is substituted with lysine, the positions 25 to 390 of SEQ ID NO: 7 except that the glutamic acid at the position 38 of SEQ ID NO: 7 corresponding to the position 38 of SEQ ID NO: 2 is substituted with lysine, the positions 25 to 390 of SEQ ID NO: 8 except that the glutamic acid at the position 38 of SEQ ID NO: 8 corresponding to the position 38 of SEQ ID NO: 2 is substituted with lysine, or the positions 25 to 390 of SEQ ID NO: 10 except that the glutamic acid at the position 38 of SEQ ID NO: 10 corresponding to the position 38 of SEQ ID NO: 2 is substituted with lysine.
 8. The method according to claim 1, wherein the γ-glutamyl group donor comprises L-glutamine or a salt thereof.
 9. The method according to claim 1, wherein the reacting of Val-Gly with the γ-glutamyl group donor occurs in the presence of a microorganism containing the γ-glutamyltransferase, and the microorganism containing the γ-glutamyltransferase is a bacterium belonging to the family Enterobacteriaceae.
 10. The method according to claim 9, wherein the microorganism containing the γ-glutamyltransferase is an Escherichia bacterium.
 11. The method according to claim 10, wherein the microorganism containing the γ-glutamyltransferase is Escherichia coli.
 12. The method according to claim 9, wherein the microorganism containing the γ-glutamyltransferase is deficient in peptidase D.
 13. The method according to claim 1, wherein the reacting of Val-Gly with the γ-glutamyl group donor occurs in the presence of a metal chelating agent.
 14. The method according to claim 1, wherein the reacting of Val-Gly with the γ-glutamyl group donor occurs in the presence of a microorganism containing the γ-glutamyltransferase or in the presence of a processed product of the microorganism, and the processed product of the microorganism has the γ-glutamyltransferase and is selected from the group consistent of the disrupted microorganism, an extract of the microorganism, a partially purified product of the microorganism, the purified γ-glutamyltransferase, the microorganism immobilized in acrylamide or carrageenan, and the γ-glutamyltransferase immobilized on a solid phase.
 15. The method according to claim 1, wherein the small subunit has the amino acid sequence of SEQ ID NO: 13 except that the tyrosine at the position 54 of SEQ ID NO: 13 corresponding to the position 444 of SEQ ID NO: 2 is substituted with aspartic acid, glutamic acid, asparagine, or alanine.
 16. The method according to claim 2, wherein the glutamic acid at the position corresponding to position 38 of SEQ ID NO: 2 is substituted with lysine.
 17. The method according to claim 3, wherein the glutamic acid at the position corresponding to position 38 of SEQ ID NO: 2 is substituted with lysine.
 18. The method according to claim 1, wherein the small subunit has the amino acid sequence of the positions 391 to 580 of SEQ ID NO: 3 except that the tyrosine at the position 444 of SEQ ID NO: 3 corresponding to position 444 of SEQ ID NO: 2 is modified by a substitution, the positions 392 to 581 of SEQ ID NO: 4 except that the tyrosine at the position 445 of SEQ ID NO: 4 corresponding to position 444 of SEQ ID NO: 2 is modified by a substitution, the positions 388 to 577 of SEQ ID NO: 5 except that the tyrosine at the position 441 of SEQ ID NO: 5 corresponding to position 444 of SEQ ID NO: 2 is modified by a substitution, the positions 391 to 580 of SEQ ID NO: 6 except that the tyrosine at the position 444 of SEQ ID NO: 6 corresponding to position 444 of SEQ ID NO: 2 is modified by a substitution, the positions 391 to 580 of SEQ ID NO: 7 except that the tyrosine at the position 444 of SEQ ID NO: 7 corresponding to position 444 of SEQ ID NO: 2 is modified by a substitution, the positions 391 to 580 of SEQ ID NO: 8 except that the tyrosine at the position 444 of SEQ ID NO: 8 corresponding to position 444 of SEQ ID NO: 2 is modified by a substitution, the positions 400 to 589 of SEQ ID NO: 9 except that the tyrosine at the position 453 of SEQ ID NO: 9 corresponding to position 444 of SEQ ID NO: 2 is modified by a substitution, the positions 391 to 580 of SEQ ID NO: 10 except that the tyrosine at the position 444 of SEQ ID NO: 10 corresponding to position 444 of SEQ ID NO: 2 is modified by a substitution, the positions 392 to 581 of SEQ ID NO: 11 except that the tyrosine at the position 445 of SEQ ID NO: 11 corresponding to position 444 of SEQ ID NO: 2 is modified by a substitution, or the positions 392 to 581 of SEQ ID NO: 12 except that the tyrosine at the position 445 of SEQ ID NO: 12 corresponding to position 444 of SEQ ID NO: 2 is modified by a substitution.
 19. The method according to claim 1, wherein the large subunit has the amino acid sequence of: the positions 26 to 390 of SEQ ID NO: 3 except that the glutamic acid at the position 38 of SEQ ID NO: 3 corresponding to the position 38 of SEQ ID NO: 2 is modified by a substitution, the positions 26 to 391 of SEQ ID NO: 4 except that the glutamic acid at the position 39 of SEQ ID NO: 4 corresponding to the position 38 of SEQ ID NO: 2 is modified by a substitution, the positions 26 to 387 of SEQ ID NO: 5 except that the glutamic acid at the position 35 of SEQ ID NO: 5 corresponding to the position 38 of SEQ ID NO: 2 is modified by a substitution, the positions 25 to 390 of SEQ ID NO: 6 except that the glutamic acid at the position 38 of SEQ ID NO: 6 corresponding to the position 38 of SEQ ID NO: 2 is modified by a substitution, the positions 25 to 390 of SEQ ID NO: 7 except that the glutamic acid at the position 38 of SEQ ID NO: 7 corresponding to the position 38 of SEQ ID NO: 2 is modified by a substitution, the positions 25 to 390 of SEQ ID NO: 8 except that the glutamic acid at the position 38 of SEQ ID NO: 8 corresponding to the position 38 of SEQ ID NO: 2 is modified by a substitution, or the positions 25 to 390 of SEQ ID NO: 10 except that the glutamic acid at the position 38 of SEQ ID NO: 10 corresponding to the position 38 of SEQ ID NO: 2 is modified by a substitution. 